Had the Red Sox known what kind of game Brad Penny would have pitched, Julio Lugo would not have been the starting shortstop on Friday.

The Sox have been putting Nick Green in the lineup on nights when they feature starters who elicit grounders. Indeed, Green – whose range is currently considered superior to that of Lugo – will be the Sox’ starting shortstop on Saturday against the Rangers, when Jon Lester and his bat-shattering cutter are on the hill.

But Friday seemed like an opportunity when Lugo’s poor range could be masked. Brad Penny took the mound having allowed three fly balls for every two grounders, one of the most air-oriented ratios in the majors.

For a night, however, Penny found an arm slot on the mound that had the ball diving, and the Rangers were pounding the ball into the ground. Of the first 13 outs he recorded, seven were by grounder, four by strikeout and just two were recorded in the air. It was precisely the sort of outcome that the Sox were hoping to see, a sign that Penny was on his game.

“He was leveraging the ball downhill with some pretty good movement. That was actually kind of nice to see,” said Sox manager Terry Francona. “When he’s good he can throw the ball downhill with some pretty good velocity and that’s what I think we’re starting to see.”

But that trait carried with it unintended consequences. With grounders scattered everywhere, Lugo’s limited range became a glaring – and, perhaps, game-changing – problem for the Sox in a 5-1 loss to the Rangers.

Penny, who blitzed through the first four innings, found himself in his first jam in the fifth. But he seemed to work out of the situation. After putting runners on first and second with one out, he got a fly ball out from Jarrod Saltalamacchia and then what seemed, off the bat, like it might be an inning-ending grounder from Elvis Andrus.

The ball appeared playable — no more than a couple steps to the right of shortstop Lugo. But Lugo’s steps towards the ball were short and hesitant, and his attempt to spear the ball with a dive proved inadequate.

It wasn’t necessarily a routine play, but a good read of the ball off the bat likely would have resulted in the third out of the inning. Instead, the ball bounded through to left field, scoring the first run of the game, and prolonging the inning.

Penny could not mask displeasure. On the mound, he bent, putting his hands on his knees in a pose of frustration. After the game, he admitted that he lost focus at that moment, though he insisted that his disappointment was in himself, rather than in Lugo.

“I just got ahead of myself, lost focus for a little bit and made a couple bad pitches,” said Penny. “It wasn’t a great pitch (to Andrus) being in a 3-2 count. There were I few times where I was at 3-2, where (the runners) would start. You can’t get into that situation. It just kills you as a starting pitcher. (The frustration) wasn’t about Julio. I left a ball up to a guy I think I should get out.”

Yet while Penny tried to say that he was merely frustrated with himself, the explanation seemed like an act of revisionism. Regardless, Penny – by his own admission, lacking focus – left a 2-0 fastball to Ian Kinsler over the middle of the plate, and Texas’ leadoff hitter blasted a three-run homer into left field, and the Red Sox suddenly found themselves in a 4-0 hole.

Lugo was not charged with an error. All of the runs were charged to Penny as earned. Nonetheless, a play that a good defensive shortstop would have turned into the third out of the inning instead turned into a key moment in a four-run rally.

Then, in the sixth, a similar play occurred. With two outs and a runner on first in the sixth, Lugo failed to come up with a grounder up the middle and to his left, his dive ultimately proving inadequate. Again, this was a ball that a shortstop with average range would have turned into an out; Lugo did not, and the ball dribbled into center for a single.

And so, instead of another concluded inning, Penny had a two-out situation with runners on first and second. That turned into another run when Chris Davis slammed a ground-rule double into the right-field grandstand to put the Rangers up 5-0.

Lugo was not solely at fault for the loss. Penny should not have allowed himself to respond to the failed plays in the field by leaving balls over the middle of the plate. The Sox’ offense, meanwhile, was utterly shut down by Rangers starter Kevin Millwood, who allowed just one unearned run in seven innings. And while the Fenway fans hounded Lugo with boos, the Sox shortstop had his advocates.

“Those were always going to be hits. The guy’s diving for the ball and trying to catch it. What else can you do? It’s not like he’s not hustling,” said David Ortiz. “The guy had surgery on his knee. He’s trying. He didn’t miss a ball that anyone else could dive and get, did he? He’s maybe four steps away from a hard hit ball. There’s not much you can do about it, other than knock it down. When you’re diving for a ball, it’s not your choice.”

Yet time and again this year, Lugo simply isn’t coming up with balls that other fielders would get. John Dewan’s Fielding Bible analyzes video data to measure how many plays a player converts into an out compared to an average fielder at his position.

Lugo, according to Dewan’s measurement system, entered last night having made 10 fewer outs than an average shortstop would have converted on the balls hit to him, a staggering total given that he had played just 24 games at the position. After Friday, Lugo’s rating (32nd among major-league shortstops) will take another hit.

That is why the Sox are having Nick Green start whenever they think that a day will likely feature a wealth of groundballs. And that is a major reason why the Sox had their four-game winning streak snapped on Friday, in their return to Fenway.

Here are four other lessons learned from Friday’s loss to the Rangers:

JED LOWRIE’S RECOVERY IS A BIG DEAL

Lugo’s struggles have made him a source of frustration to his teammates (witness Penny’s slumped shoulders) and to Red Sox fans. But Nick Green – a career second baseman – isn’t exactly a Gold Glover at short either. Green, according to Dewan’s system, has been slightly below average at the position, making two fewer plays than an average shortstop (a mark that is 24th in the majors).

All of that being the case, the return of Jed Lowrie from his left wrist surgery represents a potentially pivotal reshaping of the Sox. A year ago, Lowrie was well above average at short, making no errors and positioning himself well enough that he made eight more plays than the average player at his position. If he can return to that form, he could stabilize a Sox infield that has been leaking hits.

Lowrie took what he described as “a good first step” in his return on Friday. For the first time since undergoing the surgery in mid-April, he took batting practice. Lowrie said that he was fatigued at the end of his B.P. session, something that was completely expected, and expressed enthusiasm about how he feels ever closer to returning to the field.

“There’s a focus on getting back as soon as possible,” said Lowrie. “As far as I know, everything has gone really well. I’ve been happy with how everything has been going, and I think everyone else is very happy with how things are going.”

Lowrie is trying to avoid putting timetables on his return, for fear that he would either start rushing (and potentially undermining) his rehab or that he would subject himself to disappointment should he not be back on the field according to a schedule. But clearly, as he nears the seven-week mark of his recovery, he is antsy to return to the lineup.

“Hopefully, a couple weeks. But it’s hard to say. Really, I don’t know (a timeframe),” said Lowrie. “It’s hard for me to be here. For me to sit here and watch these games where I know I’m not going to be involved, it’s hard. I try to stay out of people’s ways. Sometimes it’s hard for me to just sit down and relax, especially when I have a feeling of wanting to be out on the field. I feel like I’ve done a good job of just focusing on what I need to do day to day. It’s getting closer.”

Lowrie is not the only one who is eager for his return. The Sox are watching him closely, treating every measure of his progress as cause for optimism.

“I don't know how long this will take...but he's actually coming pretty quick," said Francona. “I was actually pretty surprised watching him on the field, how strong he looked swinging the bat.”

BRAD PENNY’S VALUE IS ON THE UPSWING

In one sense, it was a bad night for Brad Penny. The starter allowed five earned runs – his most since April 17 – while his ERA ballooned to 5.85. At this point, every run allowed by Penny will cost him money this offseason, when he reaches free agency.

That said, Penny looked, at times, dominant against the Rangers. Two terrible pitches notwithstanding – the homer by Kinsler and the double by Davis – the consensus seemed to be that it was Penny’s best outing of the year.

His fastball was powerful, precise and had good downward movement for most of the night. His curveball was a swing-and-miss offering that buckled the legs of many Rangers hitters.

“His first four innings,” said Francona, “were as good as we’ve seen. He was throwing the ball really, really well.”

Penny, too, saw grounds for encouragement.

“Curveball was pretty good, my fastball had a lot of life to it, and I could tell that I had a lot of power,” said Penny. “I feel like I’m pretty close. Tonight was the best stuff I’ve had.”

Penny’s sharpness came at a time when he is increasingly the subject of trade rumors. With John Smoltz nearing a return to the majors (Smoltz’ rehab outing for Triple-A Pawtucket was postponed by rain on Friday, and so the 42-year-old will take the hill on Saturday night for the PawSox), Penny represents surplus, and there are rumors of other clubs’ interest in him on a daily basis.

While his line score on Friday will do little to add to his worth on the trade market, scouts in attendance could report on a repertoire that could be of use to contending clubs.

KEVIN MILLWOOD OFFERED AN INTERESTING EXERCISE IN HYPOTHETICALS

In the days after Johnny Damon signed with the Yankees around Christmas of 2005, the Red Sox turned their attention to pitching. The team made a run at Kevin Millwood, the reigning American League ERA champ, in hopes of deepening its starting rotation.

The Sox’ courtship was genuine, as the team brought Millwood to Boston to meet with team brass and manager Terry Francona. Ultimately, however, the Sox were reluctant to go beyond three guaranteed years, while the Rangers extended an offer that included a four-year, $48 million guarantee as well as a vesting option for a fifth season, and so Millwood moved to Texas.

In one sense, Millwood has given the Sox little reason to regret the turn of events. He entered last night 39-40 with a 4.68 ERA with Texas. But while he had been more or less a league average pitcher over his first three years with the Rangers, in his fourth year – the one that the Sox refused to guarantee, and that Texas did – he has been outstanding.

Millwood is 5-4 this year, but with a 2.96 ERA that is tied for seventh in the American League. Moreover, with his excellent work on Friday (7 innings, one unearned run on seven hits), he is now 4-1 with a 2.90 ERA in eight career starts at Fenway.

Since 1997, when he broke into the big leagues, Millwood’s Fenway ERA is the second lowest (to Roger Clemens: 2.84) of any visiting pitcher with at least eight starts in Fenway. That success in Boston, of course, lends itself to interesting conjecture about how Millwood might have fared had he been a member of the Sox.

“I like this ball park. There’s always a lot of energy here,” Millwood said after Friday’s game. “Obviously, they’re not cheering for me but when there’s a lot of energy we feed off it, too. It just makes it a fun place to pitch.”

DUSTIN PEDROIA MIGHT NOT BE THE BEST SECOND BASEMAN IN THE AMERICAN LEAGUE

The story is now well known. When Dustin Pedroia went to Arizona State, he outplayed teammate Ian Kinsler and emerged as the ASU starting shortstop. Kinsler, in order to ensure that he would play, transferred to Missouri.

Pedroia has since been one of the most decorated baseball players in the country, both in college and in the majors. Pedroia has paperweights galore thanks to a 2007 Rookie of the Year, a 2008 MVP trophy and a Gold Glove as a second baseman.

Kinsler has none of those trophies. But he is nonetheless making a case that he is a worthy competitor of Pedroia’s for the title of the best second baseman in the American League.

Pedroia is widely regarded as the superior defender. Even though Kinsler (after a wretched defensive year in 2008) is performing well in the field in 2009, few would claim that he is better than his former ASU teammate. But among offensive measurements, one can make a compelling case for either Kinsler or Pedroia as a more dominant player.

Since the start of the 2007 season, Pedroia is hitting .323 with a .383 OBP and .465 slugging mark. He is averaging 13 homers and 17 steals per 162 games.

Kinsler, who delivered the game-changing three-run laser into the Monster Seats on Friday (his 15th homer of the year), is hitting .290 with a .365 OBP, .493 slugging, and 162-game averages of 28 homers and 32 steals since the start of the ’07 campaign.

Both players have a healthy respect for each other.

“He’s been hitting ever since he got up here. He’s got a lot of pop in his bat. He puts together a good at-bat every time he’s up there and drives the ball,” said Pedroia. “He’s a big part of their lineup. That’s why they’re heading in the right direction.”