NEW YORK -- Carl Crawford became a $20 million a year player because the baseball industry had come around to value a player’s complete skill set. True, he was not a traditional slugger who piled up 30 or 40 homers a season, but the Red Sox saw in the outfielder a player capable of impacting the game with his bat, legs and glove.

The glove was a particularly significant factor. For years, Crawford had been considered one of the best defensive players in the game, good enough to earn the first Gold Glove of any American League left fielder in 30 years in 2010. When the Sox signed him, they expected Crawford to provide game-changing defensive coverage in left field, helping to blanket the outfield.

“You couldn’t hit a double against him,” Sox GM Theo Epstein said at the press conference introducing Crawford as a Red Sox. “We’d drive balls in the gap and you like to be rewarded for those type of swings and he’d just put the ball in his pocket and it’d be an out. That was frustrating.”

This year, with Crawford in a Red Sox uniform, the frustration has been different. The left fielder has fallen well short of the tremendous standard he set with the Rays.

The latest demonstration came on Saturday, in the Red Sox’ 9-1 loss to the Yankees. With one out and the bases loaded in the bottom of the second inning and New York up just 1-0, Russell Martin manifested his hatred of the Red Sox with a soft liner towards the line in shallow left. Crawford broke well on the ball and appeared to have it in his sights, but when he went into a slide to try to catch it, the ball clanked off his glove.

“I thought I had a play on it, but I didn’t,” Crawford rued. “I just tried to make a play at it. I didn’t make it. That’s pretty much it.”

The play ended up being enormous. Had Crawford caught the ball, the Sox would have had an easy double play, as Andruw Jones had long since left second base and ventured almost all the way to third; he would have had no chance of returning to the bag. Instead, the ball dropped, two runs crossed the plate, and three more would soon follow when Derek Jeter blasted a three-run homer to right-center.

The Sox could have emerged from the inning trailing just 1-0. Instead, they were down 6-0 after two innings, and Jon Lester’s day was nearly done. Boston was likely spiraling towards its 17th loss in 22 contests in September anyway -- after all, the offense did nothing -- but it was a play that in some respects symbolized the inability of the Sox to deliver the sort of timely plays needed to win during their shocking slump.

“I think it would have been a good play but you know, it was probably above his ankles and probably was bouncing,” said Sox manager Terry Francona. “We didn’t make the play. Sometimes it’s not an error. But sometimes we need plays to be made and good teams take advantage of that and they did.”

It was a single moment that crystallized what has been a year-long phenomenon. Crawford, who was brought to Boston in no small part because of his superstar-caliber defense, has been no better than ordinary in left field.

In his salad days in Tampa Bay, scouts were wowed by the amount of real estate Crawford patrolled with Tampa Bay. Defensive metrics likewise suggested that Crawford was able to impact the game defensively as few others. In the John Dewan’s Plus/Minus defensive ratings, Crawford led all major league leftfielders in outs produced and runs saved from 2008-10. He was, similarly, a darling of the UZR statistic on Fangraphs.

This year? His UZR is that of a below average defender, as Fangraphs suggests that Crawford has actually cost the Sox 1.3 runs this year with his glove compared to an average defensive left fielder. After saving an average of 26 runs per season in Plus/Minus over the last three years, the same system grades Crawford as having cost the Sox four runs defensively in 2011, a mark that would rank him 26th among his peers, as one of the worst defensive left fielders in the majors.

Two major league sources had a slightly more favorable view of the outfielder, suggesting that their defensive metrics (when adjusting for the unusual dimensions of Fenway, among other factors) graded Crawford as roughly an average to very slightly above average defensive left fielder.

It has long been said in baseball circles that defense doesn’t slump, but the notion is simply untrue. Players are prone to swings in their defensive performances, whether as a result of injuries (and it is notable that Crawford was sidelined for a chunk of this year with a hamstring injury), a loss of confidence that can permeate all aspects of his game or something else. It is for that reason that baseball teams and statisticians believe that defense should typically be measured over a three-year period rather than just being viewed through the prism of a single season.

Even so, looking solely at Crawford’s first year in Boston, even the most generous assessments of his glove work would represent a player whose performance has been far below expectations. Instead of being a game-changing defensive player who does not allow the ball to hit grass in left field, Crawford’s failure to make plays has been more notable this season than his ability to convert them.

The assessment is harsh, given that some of the plays that Crawford does make – in particular, when showing his tremendous range to left-center and standing under a ball that slower defenders would not run down – are not the stuff of highlight reels. Still, there have been noteworthy and immensely costly misplays that have contributed to the idea that Crawford has been something other than an impact defensive player this year.

And that, in turn, has been part of a horrible first year with the Red Sox in which Crawford has managed to fall short of expectations in every facet of the game.

After a 2010 season in which he won the AL Silver Slugger Award as the best hitter at his position, he is now hitting .259 (99th among 147 qualifying big league hitters) with a .295 OPB (132nd), .411 slugging mark (95th) and .706 OPS (109th).

His defense, once surpassing, has been merely adequate.

As for the game-changing speed that used to haunt the Red Sox? Also rarely in evidence. In his first eight seasons as an everyday player from 2003-10, Crawford averaged 50 steals a year, leading the American League in the category four times. This year, Crawford has 18 steals, his lowest total in any full season in his career.

“It’s been very frustrating. That’s a big part of my game,” Crawford acknowledged prior to Saturday’s game, when he suggested that spending almost all of the year in the lower half of the batting order had limited his stolen base opportunities. “To have that taken away from you is, you know, it’s kind of tough to deal with.”

That has been true of nearly everything about Crawford’s game in his first year with the Red Sox, something that, in turn, undermined one of the key factors that convinced the team to pursue the outfielder this winter. The Sox liked Crawford in no small part because he was in the middle of his prime seasons.

Rather than paying him solely for past performance through his years of decline, the Sox would capture some of the best years of Crawford’s career. This year has been his age 29 season (he turned 30 on Aug. 5), typically around the mid-point of a player’s peak.

And yet it has been a season so far below expectations that, in some respects, it has represented a waste of a prime year. It is a season for which the Sox were willing to pay a premium; it is one in which Crawford has performed at a discount.

Of course, it would be easy to condemn the Crawford contract as being a bust based on the evidence of 2011. However, it would also represent a premature rush to judgment on a player who still has six years left on his deal with the Sox.

Even the 2011 season may not yet be lost. Crawford, who is 10-for-20 in his last six games, was moved up to second in the lineup on Saturday because the Sox are hoping that he might provide an offensive spark and to feel more at liberty to use his legs and, in Crawford’s own words, “cause a little more havoc.”

If, over the next five games, he helps to energize the Sox and nudge them into the postseason at a time when they are reeling, the contribution even in a small sample size would be significant. Or, if the Sox are playing in October, then a few well-timed swings could change the assessment of the outfielder, just as was the case for J.D. Drew in 2007, when he struggled through a terrible first season with the Sox after signing a five-year contract, but delivered some huge postseason hits to help the Sox win the World Series.

Moreover, regardless of what happens this year, the Sox believe that Crawford’s makeup is such that the 2011 season will eventually prove an aberration.

“It’s hard to explain why [Crawford has struggled]. Obviously, if we could pinpoint the exact reason why it happened someone would have done something to address it by now,” said Sox GM Theo Epstein on Friday. “I’ll say this – he’s never stopped working his tail off. He’s never stopped fighting. He’s shown accountability. He’s taken responsibility for the year he’s had. He’s come out publicly, in a couple forums and said he’s had a bad year. That’s a great sign, a great indication, that he’s going to bounce back from the year he’s had.”

Nonetheless, the fact that the Sox are looking for Crawford to rebound rather than having an opportunity to celebrate his contributions and the skills that led them to sign him is telling. There may yet come a time when Crawford is viewed as an impact player in Boston, but it hasn’t come yet.