Just how damaging was Jacoby Ellsbury’s 2015 knee injury?

When Jacoby Ellsbury was inked to a seven-year, $153 million deal with the Yankees prior to the 2014 season, it was understandably met with some skepticism. Perhaps the most common criticism came in the form of a rhetorical question: "The Yankees just signed an injury-prone speedster to a long-term contract through his age-37 season...what could possibly go wrong?"

Since then, the critics' fears have been realized. Ellsbury has lost a step or two on the basepaths and in the field, but the injury concerns are what have most damaged Ellsbury's career in New York. Although he hasn't missed a ton of time due to ailments, a strained right LCL sidelined him for two months in 2015 and may have caused a nose-dive in performance.

Ellsbury's injury on May 19^th of last season didn't look overly severe; in fact, he remained in the game after meeting with the team trainer and Joe Girardi on the field. Things aren't what they always seem to be, though, and this injury was clearly much worse than anyone could have imagined.

Endpoints are considered arbitrary, but using May 19^th as the ‘turning point' isn't arbitrary at all. Before exiting the game with a knee injury, Ellsbury was hitting .324/.412/.372 over a month-and-a-half of baseball. He had a 124 wRC+, 14 stolen bases, and looked to be on his way to a second straight impressive campaign with the Yankees. The previous year, Ellsbury was worth 4.0 fWAR, hitting .271/.328/.419 with 16 home runs and 39 stolen bases.

After returning from the disabled list, Ellsbury hit .223/.271/.331 over the remainder of the season. In twice as many at-bats than before the injury, the former Red Sox stole half as many bases and saw his pre-injury wRC+ of 124 sliced in half. To put an emphatic close to a disappointing season, Ellsbury was benched in the Yankees' Wild Card game against the Astros. Clearly, something was wrong with Ellsbury, and it can be directly traced back to that knee injury.

This season, things haven't improved much. Ellsbury has continued to perform like a below average hitter at the plate, and while he's been more active on the basepaths (17 steals), a .266/.329/.370 line is unacceptable from a player with Ellsbury's expectations and contract. By BsR, he's having the worst baserunning season of his career, and although defensive metrics are quite unreliable, UZR and FRAA are in agreement that his defense in centerfield has steeply dropped off over the past two seasons (in other words, since his knee injury).

It's not hard to see why a leg ailment would cause Ellsbury to slow, but the devastating effect it has (seemingly) had on his bat is unprecedented. Although I can't confidently say the ailment has directly led to his struggles at the plate (correlation doesn't always lead to causation, after all), it's hard to find another source of his struggles and the drastic pre- and post-injury results are convincing enough.

Since May 19^th of last season, Ellsbury has walked less, struck out more, and hit for less power. Nothing in his batted ball numbers explain this drop in performance, and his plate discipline metrics aren't overly noteworthy either. He has made less contact outside the strike zone since the injury (78.8% of the time from 2014 to May 19^th, 2015 compared to 69.7% after), but this certainly doesn't tell the whole story.

To be honest, I'm a bit stumped by the numbers. There's no smoking gun for why Ellsbury has been so bad since the injury, but it's indisputable that he has been. I'm about as far as you can get from a swing mechanics guru (just ask my high school baseball coach), but I looked for differences in Ellsbury's swing between pre- and post-injury and found...nothing.

It feels wrong to come to the end of an article without a definitive answer to Ellsbury's struggles, but it's better than using one statistic to explain away everything when that isn't really the case. The 32-year-old has been a definitively worse hitter since getting injured, but it's hard to tell exactly why.

The most likely answer to Ellsbury's struggles is that his mechanics and momentum were thrown for a loop last season, as hitting with a compromised lower half rarely turns out well. This season, he's healthier (as evidenced by the rebounding stolen bases), but there's still something that isn't right. Maybe Ellsbury's mechanics aren't back, or maybe the center fielder simply lost strength in his knee that has kept him from playing up to his usual performance. Then again, maybe the injury has nothing to do with Ellsbury's struggles, and he has simply been bad since returning the disabled list (I don't believe this is the case, but it's technically a possibility). In all likelihood, Ellsbury still isn't right, and a variety of factors—many stemming from that knee injury—have caused him to drop to a below average hitter at the plate.