BALTIMORE – What is Jacoby Ellsbury?

At his best, it is evident what the center fielder can be. His tools give him the potential to impact the game as dramatically as almost anyone in baseball (save perhaps for Krypton native Mike Trout), something that became dazzlingly apparent in a 2011 season when Ellsbury finished second in the AL MVP race. 

But this year, that dynamic talent has rarely been seen in the fashion that made Ellsbury’s last season so captivating. He’s capable of being a 30/30 player. But his career suggests that it’s impossible to bank on such a level of performance. 

Last year, the Sox benefited from the most productive center fielder in the American League. This year, Ellsbury has been below average at the position. After going 0-for-4 with a strikeout on Sunday, he’s hitting .273 with a .316 OBP, .375 slugging mark, .692 OPS, four homers and 14 steals in 72 games. 

Those numbers are worse than the line of the average American League center fielder: .267/.328/.430/.758. They also represent career worsts for Ellsbury in all four rate stats in any season in which he has at least 100 plate appearances. 

Still, Ellsbury insisted that his decreased productivity this year was a function of the fact that his playing time has been limited due to injury, most notably the shoulder subluxation that sidelined him for three months from mid-April to the All-Star break. 

“You give me the at-bats, and my numbers will be right there,” he said on Saturday. 

(Ellsbury declined to give any insight into the nature of the recent condition that kept him out of the lineup from Sept. 20 until Saturday. “Nothing real exciting to report on. … Wasn’t a big deal,” he said.)

Yet manager Bobby Valentine suggested that Ellsbury’s down season was not necessarily a reflection solely of the injuries he’d endured.

"I didn’t see him play that way this year. He never quite hit his stride this year," Valentine said, referencing the performance gap between 2011 and 2012. "I don’t know [if injuries were at the root of the struggles]. I’ve had a lot of conversations with the coaching staff about it, a few with Jacoby about it. It’s just the feel. He hasn’t had the feel, from what I gather." 

The challenge with Ellsbury as he moves ever closer to free agency following the 2013 season is that it’s nearly impossible to say with any confidence whether he can have another year approximating his breathtaking performance of 2011. 

He has four homers this year, and he has never hit as many as 10 homers in any professional career aside from his 32-homer campaign in 2011. He remains the only player in baseball history to hit 30 homers as a 27-year-old without having hit at least 10 homers in any other season. What he did in 2011 represented, to date, a complete outlier in his career. 

Most superstars have floors that suggest that, in their worst seasons, they will still perform at an above-average level. But the five-plus seasons of Ellsbury’s career suggest that, in the batter’s box, there are few guarantees that he can be more than an average regular. Because of his defense, position and speed, he still represents a well-above-average player, but for most of his career, he’s fallen well short of elite.

His numbers from a career that saw him make his major league debut at age 23 and now has him wrapping up his age 28 season: A .297 average, .350 OBP, .443 slugging mark, .792 OPS with 56 homers (16 per 162 games) and 189 steals (53 per 162). Ellsbury has a career OPS+ of 107, meaning that his OPS (adjusted for parks) is 7 percent better than the league average. 

Such numbers still put him in respectable company but do not necessarily merit superstar status. Since 1995 (the post-strike era), Ellsbury is one of 11 outfielders who, between ages 23-28, had an OPS+ of 100-115 (meaning an OPS that rated as league-average to 15 percent better than league average) with 100 or more stolen bases in at least 2,000 plate appearances. 

Here’s the complete group:

Clearly, Ellsbury’s ability to hold his own at the plate while also introducing an element of speed on the bases and deliver quality defense merits comparison to a number of excellent players. His career totals -- elevated considerably by his incredible 2011 season -- suggest a player who has performed in a fashion that merits some comparison to the likes of Carl Crawford, Carlos Beltran, Johnny Damon and Mike Cameron, among others. 

In long-term negotiations, of course, Ellsbury and agent Scott Boras likely will play up the statistical similarities to Crawford (who signed a seven-year, $142 million deal -- you might have heard something about that -- after his age 28 season) and Beltran (who signed a seven-year, $119 million deal after his age 27 campaign). 

That said, Ellsbury falls short of both Crawford and Beltran in terms of OPS+ (Crawford had a 115 mark, Beltran a 114), power and, not inconsiderably, durability. And, while his hitting line between ages 23-28 nearly mirrors that of Johnny Damon (who signed a four-year, $32 million free agent contract with the Sox after his age 28 season), Ellsbury also falls considerably short of his Red Sox center field predecessor in terms of ability to stay on the field, having played 300-350 fewer games than that trio.

For now, it seems like a stretch to suggest that Ellsbury merits a contract in line with the deal that the Sox conferred upon Crawford as a free agent. Crawford had better numbers in terms of power, OPS, stolen bases and durability. He had just turned 29 when he reached free agency; Ellsbury will turn 30 near the end of the 2013 season, meaning that the team that signs him will get one less year of his theoretical prime. 

Moreover, Crawford had a track record of considerable consistency when he became a free agent. He’d posted an OPS of .800 or better in five of the six seasons preceding his free agency. He’d played in at least 150 games in six of his eight full big league seasons, while playing in 140 or more games in seven of eight campaigns. 

Ellsbury, meanwhile, has three seasons of 140-plus games to his credit. Aside from 2011, he does not have a full big league season with an OPS above .770. 

(Obviously, the fact that Crawford was limited to a total of 161 games in his two years in Boston suggests that it would be a mistake to assume that a healthy player will remain healthy; however, that doesn’t mean that the opposite conclusion -- that a player who is often unhealthy may struggle to stay on the field -- can be dismissed.)

It’s also worth noting that Ellsbury’s career numbers are relatively close to those of B.J. Upton (whose two-homer game on Sunday is not reflected in the chart). Upton, who will be a free agent this winter following his age 27 season -- meaning the team that acquires him will get nearly all of his prime career years -- has an OPS+ of 108 over the last six years. 

He has three 20/20 seasons in the big leagues, including this season, in which he’s slammed 28 homers and stolen 31 bases. He has a track record of demonstrable health. In other words, if placing a wager on whether Ellsbury or Upton was more likely to produce a 30/30 season during the next contract he signed, the Rays center fielder, at this point in the careers of both players, would represent the obvious bet. 

None of that is meant to disparage Ellsbury, an electrifying, game-changing player when healthy and locked in. It also merits mention that the vast majority of time he’s missed in his career has been for considerable injuries, whether his five broken ribs in 2010 or his shoulder injury this year. (That said, Ellsbury’s sensitivity bordering on paranoia about whatever recent affliction kept him out of the lineup for just over a week was vexing.)

However, it’s hard not to look at Ellsbury and conclude that there’s considerable difficulty in projecting what he will be going forward. Was 2011 a one-time-only perfect storm? Or can he again see all of the elements of his game come together to define him as one of the most valuable players in the game for years to come?

That is at the heart of the near-impossibility of contemplating a long-term deal with him, whether for the sake of kicking the tires on an extension this winter or, more likely, determining his worth as a free agent after the 2013 season. 

Jacoby Ellsbury has been a Red Sox for more than five full seasons now. Yet it remains a struggle to pin down what, precisely, he is as a player. Now 29, and one year from free agency, he remains a mystery.