Pettitte appeared to stiffen and wince after throwing his final pitch, a swinging strikeout of Kyle Seager for the second out of the fifth inning. Girardi dashed to the mound and held a fairly lengthy conversation with Pettitte, who shielded his mouth with his glove. Pettitte jogged off the field and was relieved by Shawn Kelley.
"Obviously, I'm realistic," Pettitte said. "I know how these guys are. I asked them if I could stay out there and try to get through it out there. I'm hoping it's a spasm and it settles down."
Girardi -- no stranger to injury updates in a season that has already been filled by them -- remained hopeful as well.
"I don't think Andy's [injury] is too bad," Girardi said. "His trap just kind of got a knot in it, and to me it was affecting his velocity. That's why I took him out. It's muscular, but I don't know what that means going forward."
Pettitte, 4-3 with a 3.83 ERA, said that he never felt especially sharp on Thursday, allowing two runs and four hits, with three walks and five strikeouts to take the loss.
"It was just really tight," Pettitte said. "I couldn't get extended at all. I really just felt like I was kind of cutting everything off and just really trying to throw it in there almost slower than slow, to tell you the truth."
Pettitte missed a start in April with lower back spasms, but he said that he did not believe the two injuries were related. Pettitte turns 41 next month, but he said he is reluctant to use his age as an excuse.
"There's no doubt I'm getting old, but I don't want to sit here and tell you that that's the exact reason why, you know?" Pettitte said, with a chuckle. "I saw Moose [Mike Mussina] go through a full season at age  and win 20 ballgames."
The Yankees have already had 12 players on the disabled list this season, with 10 of those men still on the shelf.
Bryan Hoch is a reporter for MLB.com. Follow him on Twitter @bryanhoch and read his MLBlog, Bombers Beat. This story was not subject to the approval of Major League Baseball or its clubs.