Under slightly different circumstances, it might have been Gio Gonzalez on the hill for the Red Sox on Saturday.

The Sox pursued the left-hander aggressively when the Athletics shopped him last offseason, with the two teams contemplating a deal that would have brought Gonzalez to Boston along with Andrew Bailey and Ryan Sweeney. In order for that deal to happen, the A’s would have required a huge prospect haul, likely including a player like Will Middlebrooks to make the deal happen.

In the end, the cost of Gonzalez was too steep for the Red Sox. The Nationals parted with a package of four prospects (Tommy Milone, now performing well in Oakland’s rotation; Brad Peacock, a starter in Triple-A; A.J. Cole, a high-ceiling pitcher in A-ball; and Derek Norris, a catcher enjoying a good year in Triple-A) for the left-hander.

The Sox’ reticence was understandable. Middlebrooks represents a potential cornerstone player for years to come, a player who won’t be eligible for free agency until at least after 2018 and who won’t start getting significant salary bumps through arbitration until at least after 2014.

“I heard about it in spring training from someone in the media. I keep my head out of that stuff and the internet. I leave that stuff to my mom,” Middlebrooks said after going 0-for-3 with a walk, his average at .310 with an .876 OPS after the Sox' 4-2 loss to Gonzalez and the Nationals. “I didn’t think I would go anywhere. I was confident of being here and helping this team win.”

He’s doing that, of course. And if indeed holding the line on Middlebrooks cost the Sox Gonzalez, it’s difficult to argue with the decision. That is particularly true given that, at the time he was being dangled, the Sox had some reservations about the left-hander, particularly due to his high walks totals in the American League West.

Gonzalez issued 92 walks in 2011 and led the league with 91 walks last year, producing his sterling ERAs of 3.23 and 3.12 in spite of those free passes. There was concern about what that might mean should he relocate to the AL East, the most formidable offensive division in baseball.

The Nationals did not have the same hesitation. They saw a potentially elite left-hander, and so far, that is what they’ve gotten, something that continued on Saturday as Gonzalez mowed through the Sox lineup for 6 1/3 innings, allowing just three hits to improve to 8-2 with a 2.35 ERA in Washington’s 4-2 victory at Fenway Park.

"Those last two guys we've faced," Dustin Pedroia said of Nationals starters Stephen Strasburg and Gonzalez, "are No. 1s."

Opponents are hitting a major league-low .168 against him; his WHIP is 1.00; he’s given up just one homer in 72 2/3 innings. He's delivered precisely what the Nationals hoped to see when they dealt for him.

“I’ve scouted him personally for many, many years, since he was in high school. He’s the same personality, same character guy that we thought we were getting,” said Nationals GM Mike Rizzo before the game. “We always thought that he was an All-Star-caliber pitcher when we acquired him, but we also thought there was upside to him.

“We thought if he could taper his walks down just a hair, he goes from a good All-Star-type pitcher to an elite left-handed pitcher. We had some ideas how to improve his command and location. He put a lot of work into it this offseason and spring. We didn’t need to have a big transformation, but we thought a tweak would put him into a different class. He’s done that.”

Gonzalez represents part of the difference in how the Sox and Nationals approached the construction of their rotations this offseason. The Sox felt comfortable with a front three of Jon Lester, Josh Beckett and Clay Buchholz; given that comfort, and the fact that they had little available payroll, they went with a low-cost approach to the back of their rotation.

Ultimately, after a spring training competition, Felix Doubront and Daniel Bard were selected to round out the rotation. Doubront (6-2, 4.34 ERA, 72 strikeouts in 66 2/3 innings) has been largely terrific. His stuff has played against all of the AL East teams. He’s put the Sox in good position to win in almost every one of his starts save for a six-run, four-inning yield to the Nationals on Friday.

Bard, however, is now in Triple-A, the experiment of his conversion to the rotation proving more challenging than expected, with no clarity about whether it will continue going forward. He will work in short stints in Pawtucket while trying to rediscover his mechanics.

In his absence, the Sox now have Daisuke Matsuzaka in the rotation. The right-hander showed swing-and-miss stuff that manager Bobby Valentine deemed “usable” on Saturday, after he punched out eight and walked one while allowing four runs in five innings. Matsuzaka factored into the Sox’ relative comfort in taking a conservative approach to rotation building this offseason, as the team felt he would come back and be able to make an impact sometime this year.

Similar factors led the Sox to pass on Edwin Jackson, the young but well-traveled free-agent right-hander who was 49-41 with a 4.06 ERA from 2008-11. The Sox bid, but did not near the one-year, $11 million offer for which Washington signed him.

Financial realities prevented the Sox from making a bigger play. Still, now with the Nats, he represents something of a grass-is-greener option. In 11 starts, he has a 3.11 ERA while striking out 7.3 and walking just 2.4 per nine innings. His addition gave the Nationals both stability and depth to a rotation that already had immense talent.

“The thought process was that we were very comfortable with the starters we had, but we knew there were several questions in our rotation,” said Rizzo. “Chien-Ming [Wang] was coming off the two arm surgeries. He pitched well at the end of last season. We had hope for him, but we weren’t positive. Jordan Zimmerman, it was his first time in a full season after Tommy John surgery. We felt good about him but knew there was a little of the unknown. And [Strasburg], we knew we were shutting him down at some point in the season, so we knew there was an innings shortage there.

“Obviously, we waited for Edwin’s numbers in terms and dollars to come into play where we felt comfortable doing it. We felt great with the character of the player, the makeup of the player and the ability level of the player. He’s fit in seamlessly.”

The first-place Nationals have been rewarded handsomely for their offseason strategy. Their 3.01 ERA is the lowest in the majors; the Sox, mired in last in the AL East, are 28th at 4.47. Washington’s starters have a 2.93 ERA, again the best in baseball; the Red Sox rotation is 27th with a 4.93 ERA.

It is easy to look back and suggest, with hindsight, that the Sox should have been more aggressive in their approach to the back of their rotation, but such an after-the-fact judgment will require more time to render. Doubront’s strikeout-an-inning stuff underscores the logic of the Sox’ approach. If Matsuzaka helps provide stability at the back of the rotation, or if Bard finds his way back, or if the team can use its remaining financial flexibility and prospect inventory to acquire a starter before the trade deadline, then perhaps the team’s rotation-building approach ultimately will prove successful.

Even so, on a day when the Sox sank back below .500, it would have been hard for the team not to look at Gonzalez -- the prize in a sweepstakes for which it was runner-up -- without at least a hint of melancholy at the thought of what might have been.