CHICAGO -- No one knows. There’s no good, concise summary to explain what is arguably the most mystifying shortcoming on the Red Sox.

By and large, Adrian Gonzalez seemed slump-proof in his first season in Boston, and for that matter, in the several seasons before that. The sweet-swinging first baseman seemingly possessed both the hand-eye coordination and the cerebral advantage in the batter-vs.-pitcher dynamic to the point where it appeared almost impossible to imagine a sustained period in which he would bring very little to the table offensively.

He has a track record of several years during which he’s been elite. Unlike the past two seasons -- during which he remained one of the top hitters in the game -- he is considered healthy. At 30 years old, Gonzalez is squarely in his prime.

Everything points to the idea that Gonzalez should be putting up monster numbers. Entering the year, it was fair to ask whether he might be able to improve on a 2011 campaign in which he hit .338 with a .410 OBP, .548 slugging percentage and .957 OPS, numbers that earned him a seventh-place finish in American League MVP balloting.

But lo and behold, 63 games into the year, Gonzalez is hitting .267 with a .318 OBP, .414 slugging mark and .732 OPS with five homers. Those aren’t terrible marks, but they are terrible numbers for Gonzalez. Both inside and outside the organization, talent evaluators admit complete bafflement about why it is that a hitter whose approach is perennially one of the best in the majors is suddenly performing in drastically atypical fashion.

He’s swinging at first pitches as never before. He’s put the initial offering of an at-bat in play 48 times thus far, on pace to do so 123 times -- or 46 percent above his previous career-high of 84. Yet whereas early contact in the past had usually been a harbinger of good things for him (he has a career average of .374 with a 1.051 OPS on the first pitch), this year, he’s hitting .277 with a .738 OPS on 0-0 counts.

“I think the biggest reason why I’m hitting .260 is putting myself in non-hitter counts,” he explained recently. “That’s an adjustment [needed] to pitchers making pitches early in the at-bat or me chasing a pitch early in the at-bat that I shouldn’t have swung at.”

The impact of getting into those “non-hitter” counts has been magnified by his struggles when falling behind. In prior seasons, he had been one of the better two-strike hitters in baseball. His career line of .208/.277/.350/.628 with one strike remaining in a plate appearance represented a well above-average mark. But in 2012, he has a .145 average, .211 OBP, .191 slugging mark and .402 OPS with two strikes -- career lows in each of those categories.

The net result? Whether early or late in the count, this is not the same hitter who has been one of the best in the majors since 2006, with one notable exception.

With runners in scoring position, Gonzalez is hitting .369 (6th in the AL) with a .410 OBP, .600 slugging mark (7th) and 1.010 OPS (10th). Those numbers are largely in line with -- and in some respects, better than -- Gonzalez’s career marks (.320/.436/.546/.982), and have helped him to continue making an impact even at a time when his game has yet to take shape this season.

Even so, between his lack of power this year and his lack of power this year, he’s been a far less impactful player than the one for whom the Red Sox signed up when they acquired him in a trade in December 2010 and then signed him to a seven-year, $154 million extension from 2012-18. Despite an AL-best 22 doubles, he has just five homers -- a mark is tied for 71st in the American League.

A man who has been an on-base machine has been just the opposite to date. He has a .318 OBP, a mark that is 89th in the AL among hitters with at least 50 plate appearances, or slightly worse than the .325 mark posted by Nick Punto. His .732 OPS is 87th in the AL.

The consequences have been significant. Gonzalez, after all, should be the anchor of the lineup. The team entered the year thinking it had at least four elite offensive players in Jacoby Ellsbury, Dustin Pedroia, Gonzalez and David Ortiz that would ensure a robust run-scoring machine that would curtail any offensive struggles.

But Ellsbury has missed almost the entire year due to his shoulder subluxation and Pedroia has struggled since early May while dealing with his torn adductor muscle in his thumb. Ortiz has held up his end of the bargain, but Gonzalez cannot say the same.

The result has been offensive inconsistency from a lineup for which that was not expected to be an issue. The Sox are still averaging 5.06 runs per game, good for second in the American League and third in the majors.

But in the team’s last 10 games, it has scored three or fewer runs on six occasions, netting five losses. The stretch has underscored the idea that the Red Sox, by and large, have been an offense-dependent team this year. They are 3-23 when scoring three or fewer runs, a .115 winning percentage that is the fourth worst in the majors when failing to score four runs.

With Gonzalez, the Red Sox were expected to receive a performance that ranked among the best among American League first basemen. Instead, the team’s first basemen have performed at a level that is basically league average. To this stage of the year, Gonzalez has been worth about 20 fewer runs (or about two fewer wins) of offense than he was through a comparable fraction of the season last year.  

It’s all a gloomy portrait, of course, but there is at least some reason for optimism, beyond Gonzalez’s 3-for-5 night on Wednesday in Miami. He is about to play a weekend series against the Cubs in a park where he has frequently gone off.

In 19 games in Wrigley Field, he has hit .320 with a .400 OBP, 1.093 OPS and seven homers.

“It’s a small park, I’ll tell you that,” Gonzalez once observed. “I know the wind can blow in or whatever, but every time I’ve been there, it’s seemed like a good place to hit even when the wind is blowing in.”

Perhaps that comfortable setting will result in a Gonzalez who looks more familiar. Just as surely as everyone is baffled by the first baseman’s early-season struggles, so, too, does virtually everyone expect him to leave them behind.

“I think Adrian Gonzalez is a great bet,” Cubs president of baseball operations and former Red Sox GM Theo Epstein said on the Dennis & Callahan show, “to perform at an elite level the way he has throughout his entire career and the way he did for most of last season.”

It is an opinion held almost universally. The Red Sox can ill afford for it not to prove true.