Meet Adam Warren. He was born in August 1987, which means he'll turn 25 later this year. A right-handed starter drafted out of North Carolina, he is ranked by Baseball America as the Yankees' 15th-best prospect overall. This is what BA has to say about him:
Warren went 32-4 in college...and four of his 2009 teammates already have reached the major leagues. He had a breakthrough year in 2010 (...) but a modest 2011 campaign in his first run at Triple-A (...) He didn't get a win after June 20 (...) even though his peripheral stats improved (...) Warren pitches off his 89-93 mph fastball and touches 94-95 fairly regularly (...) Warren profiles as a back-of-the-rotation starter, and for the Yankees he's more important as depth than as a key part of their future.
I'm sure you, the astute baseball fan, can see the problem Warren has: profiling as a back-of-the-rotation starter at best for a team that can afford to keep multiple front-end starters in its rotation means that chances to break in with the team (never mind having a breakout with it) almost become limited to worst-case scenarios.
The Yankees, of course, can't be blamed for doing everything they can to make theirs the best possible team it can be. Thus, with six rotation candidates (CC Sabathia, Michael Pineda, Ivan Nova, Hiroki Kuroda, Freddy Garcia, Phil Hughes), the club found a taker for A.J. Burnett, and just re-signed Andy Pettitte. Pettitte was signed to a minor-league contract, but make no mistake; if a pitcher who threw the majority of 15 seasons with the Yankees, who has been involved in every World Series the team has won since 1996, is physically able to pitch, he'll be in a major-league uniform as soon as he's ready.
Or, put it this way: Freddy Garcia went 12-8 with a 3.62 ERA for the Yankees last season, and now there is a case being made that if Pettitte does manage to come back all the way (even if everything goes to plan, the soonest Pettitte would be back in the Bronx would likely be in May, allowing six weeks for a spring training of sorts), that the Yankees should trade him. Too much pitching depth, it would seem, is a wonderful problem to have...
...unless, of course, you are that depth. In which case, you might not see the light of day. At least, not without some major trade or serious injury opening up a spot for you. As BA suggests, Warren's chance with the Yankees will probably come in long-relief, which really isn't a problem except in that whoever doesn't make the Bombers' rotation is probably slated for the bullpen, and that's not a job they'd be likely to lose any time soon (look at Boone Logan).
Warren isn't the only one of the Yankees' minor-league pitchers not named Banuelos that might be expected to make his major--league debut at some point this season. Dellin Betances (RHP) might be the most familiar name, while David Phelps (RHP) seems to almost always be mentioned in the same breath as Warren (even BA's capsule states "Phelps resembles Warren in that he profiles as a back-of-the-rotation starter..."). Phelps has the added disadvantage of being a year older than Warren—meaning he'll turn 26 after the season—not old per se, but the window in which Phelps will have an opportunity to prove his worth on the big stage will continue to grow smaller with each passing season.
The Yankees (and their fans) have a lot of reasons to be excited about Pettitte's change of heart: a cheap, team-friendly contract and the return of one of the "core four" members (which, perhaps, seems to lead credence to the belief this will be Mariano Rivera's last season, given his lobbying of the left-hander to reconsider his retirement). If Nova or Pineda has a sophomore slump, if someone gets hurt, or if one of those assumed to have a rotation spot is simply ineffective, Pettitte becomes a fantastic insurance policy—certainly, at least, a better one than a pair of minor league pitchers who have yet to make their major-league debuts.
This leaves the Yankees with a bit of a quandary: is it more beneficial to keep Warren and Phelps around as potential break-glass-in-case-of-emergency relief, or to attempt to trade them and get something in return that might be more useful to the team's immediate needs? It goes without saying that neither has the ceiling of a Jesus Montero or Manny Banuelos, so there won't be a Michael Pineda-esque return, but if the Yankees can't (or don't want to) use them, there's probably another team out there that could. Warren and Phelps are more than organizational filler, and even if they end up as back-end starters or long relievers for a major-league team, it's still a major-league job requiring major-league skills. If the Yankees go too much longer without either bringing them up or trading them, there becomes a distinct possibility that any potential value they might have becomes lost.
On the other hand, a team almost never makes it through the entire season without someone getting hurt and missing time, often as not this player happens to be a pitcher. If it's a relatively minor injury, with a pitcher expected to miss only a game or two, then it might make sense to give Warren or Phelps his chance; if he succeeds, he'll probably get another one, if not, well, it was only one start, and it will always be cheaper to solve problems in-house than any other way.
As a team, the Yankees are in a fortuitous situation. They've got pitchers like there's water in the ocean, and a lot of good pitchers, too. For a team that entered the offseason with pitching considered it's biggest weakness, the Bombers have certainly come a long way—but whether Adam Warren or David Phelps will see it that way is another matter entirely.