Around the time the Yankees were shopping for a couple of outfielders in the 2013-2014 offseason, FanGraphs published an article, "Jacoby Ellsbury's Excellent Aging Curve," in which Dave Cameron said the following:
"The idea that Ellsbury-type players fall apart as their speed declines is simply exposed as a myth. Players like this that have come before have simply made adjustments to compensate for their declining speed, and have continued to produce at a high rate even as their speed and defense diminished. Don’t buy into the idea that Jacoby Ellsbury is headed for a crash as he slows down. In fact, if he performs like the average of the similar players who came before him, the rumored price tags of $120-$140 million might end up proving to be a bargain."
It may seem like something to chuckle at in 2016, but it wasn't a crazy concept. Ellsbury to that point had a couple of freak injuries on his ledger, but he did not show a persistent record of nagging injuries. The Yankees generally believed this, too, as they signed the center fielder to a seven-year, $153 million contract that was immediately, and to this day, described as an overpay.
And it pretty much was. The Yankees easily beat the Red Sox's second-best offer, and other than his monster 2011 performance, his value was largely tied up in his defense, something you'd expect to go downhill almost immediately upon putting on pinstripes. Year one was what we wanted: 3.5 WARP/4.0 fWAR/3.3 rWAR, depending on how good you thought his defense was. In year two, it was a disaster. He played just 111 games because of a right knee sprain that not only forced him to miss time, but also severely impacted his play when he came back, and he hit just .257/.318/.345 with seven home runs.
And now just this Saturday, we got yet another Ellsbury injury scare as he took a pitch off of his right wrist. He went for a CT scan and it came back negative, so luckily he should be ready for Opening Day. But, what if it was more serious?
Luckily for this team, there is actually a backup plan. While in years' past (see: 2013) a major injury left a hole, the team now has a fourth outfielder in Aaron Hicks that could fill in if need be. If Ellsbury were to miss extended time, Brett Gardner would become the de facto center fielder, and Hicks the everyday left fielder. The drop off from this duo wouldn't be as great as--say, Brett Gardner and Mason Williams/Slade Heathcott--but it would still be significant.
According to Baseball Prospectus' depth charts, Jacoby Ellsbury is the second best player on the team by projected WARP (3.2; the highest is Brian McCann's 4.0). This assumes he gets 85% of center field playing time, and Gardner would get 75% of playing time at left field and 10% of playing time at center field. Assuming an injury, Gardner would get a chunk of that center field playing time, and Hicks a larger chunk of left field playing time. The effect, cumulatively, is unclear.
It all depends on how you interpret each outfielder's positional defense, but a good estimate, based on Baseball Prospectus' FRAA via PECOTA projections, is that the total defensive drop off between Ellsbury/Gardner and Gardner/Hicks would be something like this, scaled to 162 games:
It's not that big of a gap, but it's not non-trivial. This is obviously predicated on full playing time, but you get the idea. The more games Ellsbury misses, the more this difference gets amplified; while a wrist injury would only sideline him for a month, anything like last year could cost the Yankees a half of a win on defense alone. And when you consider that Ellsbury is a .265 True Average hitter while Hicks is a .254 True Average hitter, the gap looks a bit wider.
All of this is to say that losing Ellsbury would be devastating to a team operating on such shaky ground to begin with. The rotation has a ton of question marks, there are six position players over 30, and the AL East boasts at least two teams that are as good or better right now. If you lose a win here or a win there, you can find yourself on the sidelines come October. Luckily the Yankees have themselves Aaron Hicks as a band-aid if worst comes to worst, but it's not ideal. In a baseball world where they need 150 games of Good Jacoby Ellsbury, the Yankees organization is breathing a sigh of relief.