I've watched the release of Oliver Perez and Luis Castillo across town with a lot of interested, because I think the Mets are learning a lesson that I hope the Yankees' management team understands more intuitively.
As general manager, my players have to walk a fine line: they all had better want to play, they had better come prepared to play, and they had better perform when they play; on the other hand, they had better be willing to sit when the manager doesn't need them, and they had better be happy to take a back seat to any other player for whatever reason the manager and I feel is legitimate.
So, the real x-factor for me in choosing the Yankees 4th and 5th starters, especially because the performance of Ivan Nova, Freddy Garcia and Bartolo Colon have been nearly indistinguishable, is knowing which guys are most likely to take it gracefully if they are pushed from the rotation by a midseason trade or call-up (though, based on Cashman's track record, I seriously doubt that the Yankees would bring up a minor leaguer if all 5 starters were pitching well).
I don't have a strong insight on this. I know Freddy Garcia has publicly stated he would accept a bullpen role, and I know Ivan Nova has pitched from the pen. Only the coaching staff can guess which guys could best handle a role change. I'm running out of guesses, and I've changed my mind several times. For now, I'll stand by my prediction that, barring injury, Garcia and Colon will start the season in the rotation.