Pettitte, 40, said on Sunday in published reports that he had pushed himself too hard during the team's trip to Seattle and that he would have to back off so the swelling could alleviate.
Girardi said that the Yankees continue to expect Pettitte to return to their rotation in September, and that this recent update doesn't constitute a change in that thinking. Doctors have been encouraged by Pettitte's healing thus far.
"We've said all along this is a 60-day injury, at least," Girardi said. "If you don't pick up a baseball and throw off a mound for at least six weeks, 60 days is pretty tough to get back by. ... As far as a setback, I wouldn't call it a setback. I think it's part of the healing process."
Yankees general manager Brian Cashman said that his thinking has not been impacted by the Pettitte update, noting, "I only get concerned when they tell me to get concerned."
Pettitte suffered a fracture of his fibula when he was hit by a Casey Kotchman one-hop ground ball during a June 27 start against the Mariners at Yankee Stadium. He came out of retirement this spring after sitting out for the 2011 season, joining the big league club in May and going 3-3 with a 3.22 ERA in nine starts.