For one thing, he's not what he was; for another, there's that second wild card.
It's a bummer about Curtis Granderson. I don't want to see any ballplayer get hurt, but, I mean, nobody seems to have a bad word to say about the guy and he's a good ballplayer. Now, though, he's out for 10 weeks with a broken arm, leaving the Yankees with a pretty big hole for at least the first month and a half of the regular season. That sucks ... for him, at least. I mean, this is a contract year for him, after all. And I bet it hurts a lot to break your arm. God knows even thinking about it makes me squeamish enough. For the Yankees though, this probably sucks a lot less than you're thinking it does.
Everything about the Yankees gets hyperbolized to a remarkable extent. Any loss is a sign of the apocalypse. Any win is Mardis Gras. The Yankees aren't just a successful franchise with a bunch of World Championships, they're the Evil EmpireTM and both the fans and the organization are proud of it. Derek Jeter isn't just a Hall of Fame shortstop; he's everything that's right with baseball and the greatest clutch player of all time. Alex Rodriguez isn't just a slugger struggling with injuries in the twilight of his career; he's a soft, selfish cheater with no respect for the game or his team. Mariano Rivera is some kind of demigod cross of Captain Kirk and the T-1000 (okay, that one is accurate). The point is, while it's jarring to lose a player like Granderson to such a traumatic-sounding injury, the actual cost is pretty low.
Now, Granderson does a lot of things well. He has good power to right field and is a perfect fit for the new Yankee Stadium. He's hit ten more homers than any other player in baseball over the past two seasons. He continues to run well and display excellent patience at the plate. He seems to be one of the most well-liked guys in baseball as well ... but Curtis Granderson is not irreplaceable.
The hard truth about Granderson is that he's far more popular than his play warrants. Despite all his homers, he ranked eighth in OPS and ninth in OPS+ out of 21 center fielders who qualified for the batting title. His batting average and on-base percentage dropped to career lows in 2012 thanks in part to a .260 BABIP (which may rebound) and to an alarming spike in strikeouts, which saw Granderson punched out in 28.5 percent of his plate appearances. The advanced defensive numbers (Total Zone, Ultimate Zone Rating, Defensive Runs Saved, and Fielding Runs Above Average) universally see him as a poor defender in center field.
Mind you, I'm not saying he's a bad player. He's far from it. But at age 32, with his platoon and strikeout problems, and his decreasing range in the outfield, he's a hell of a lot closer to the two-to-three win player he has historically been than the guy who led the AL in runs scored and RBI while hitting 41 bombs and placing fourth in the MVP race. That's especially true if he'll be playing left field, where the offensive expectations are higher, instead of center.
While it will hurt the Yankees to lose one of their better position players for all of those two to three wins above replacement, it's important to realize that we're only talking about six to eight weeks of baseball before Granderson is supposed to be back and in his usual form. Sure, the Yankees could stumble out of the gate, but if they do, is an extra win from Curtis Granderson really going to pull them out of a tailspin? Of course not. Nor is that win likely to be the difference between the Yankees making the postseason or not.
According to Baseball Prospectus's PECOTA system, the Yankees were projected to be a 92 win team as of a week ago, while the rest of the AL East was fighting it out in the mid-to-high eighties. I have a little more faith in the field than that, but even so, if we project the second wild card winners back to 1996, the first 162-game season with the current three-division format, we see that the average "fifth seed" won an average (and also a median) of 89 games. Even with Granderson shelved for a month and a half and A-Rod out for at least half the season, the Yankees are still north of that line. While that's not a guarantee that the Yankees will be playing come October, it's exceptionally likely that, even with a replacement-level replacement for Granderson, the Yankees are going to be playing in October so long as their aging roster doesn't suffer an additional cascade of injuries.
I guess what I'm trying to say is "don't panic." Other franchises go through this kind of thing all the time. Brian Cashman does not have to make some grand gesture of a trade to help the Yankees weather this storm; that won't necessarily help the Yankees, especially in such a small sample. All that will do is help you feel better, because some name you recognize is manning left field. The Yankees only have to replace Granderson in the most literal sense of that word. I mean, it would cost them a lot of games if nobody stood out there for the few weeks that Curtis Granderson is unavailable, but as long as somebody's manning his post, you'll hardly even notice that he's gone.