Pettitte does not have any injuries that have been made public, but there has been some speculation that his elbow is not 100% based on the fact that his trademark cut fastball has seldom been an effective pitch this year. In the past whenever his elbow hasn't been 100%, his cutter isn't effective and his overall pitching suffers.
He started thowing much later in the off season this year in order to give his arm some extra rest given that he pitched the second most innings of his career last year. So it's possible that he's a bit behind schedule and may break out later, but if he ends up on the DL with elbow trouble then he's obviously a bust. I'm starting him tomorrow against the Cardinals as well.