An MRI exam revealed Andy Pettitte has a small, persistent strain of the left groin, and the Yankees left-hander will not throw off a mound for a week.

Pettitte had the exam at New York Presbyterian Tuesday night after feeling his injured left groin “grab” during a bullpen session in the afternoon.

The left-hander has been out since July 18 when he strained the groin. The Yankees were hoping to have him back by Sept. 1, but it is not looking good.

After throwing 40 pitches in the bullpen today, Pettitte decided he wanted an MRI exam to see why he was still experiencing discomfort. A Yankees spokesman said it was “to put his mind at ease.”

“I’m feeling good. I’m feeling better, but when I reach back 100 percent like I’m trying to get a batter out in a game the groin area grabs,” Pettitte said through the spokesman.

Pettitte will continue to throw off flat ground.