Girardi said not to expect Pettitte back until sometime in mid September. Joe likes to guess conservatively so there is a chance Pettitte could come back at the start of September. Due to Andy's age however I would not expect him to be back until Girardi and the staff are 100% sure that he is good to go.
Though we really could use Pettitte right now due to Javy's deadarm (a skipped start would really help him out), Dustin Mosley has stepped up for now. What we really need Pettitte for is our playoff run. The man helped carry us to our 27th title last year pitching gems in all three rounds. His 18-9 record in the playoff is what is most valuable to the Yankees right now and they need to protect his arm for October.