For much of the season, Adam Warren has been one of the more reliable starting pitchers for the Yankees. Yet, with the return of Ivan Nova, the team found itself with six healthy starters, and eventually this lead to Warren being moved into the bullpen. As Jason pointed out here, this was the only option the Yankees really had, and yet it hasn't been particularly popular. Mostly because of the perception that Warren was demoted to accommodate a struggling CC Sabathia.
We don't know how much of a role Sabathia's contract played in the decision making there.