I never trusted Petitte. He's flaky and had elbow problems last year that was ominous. He's hyped in real life because he's a young lefty with a nasty cutter fastball, which is relatively uncommon. But in fantasy baseball, none of this hype means anything.
The cutter fastball has been known to destroy elbows. Ever since he had a "gap" in his elbow diagnosed last year, he's been trying to throw the pitch less frequently, which has hurt his effectiveness. It's only a matter of time before he signs up for Tommy John surgery.
Now for some good news. Of all the other Yankee pitchers, his schedule has been the toughest. There's a chance he'll do better once now that he'll be facing better matchups. I would look up some stats on a game-by-game basis to decide whether to play him or bench him.
Experience is a hard teacher. She gives the test first, the lesson afterwards.