BRONX, NY (AP) -- Andy Pettitte will back off his rehabilitation program of throwing from a mound because tests showed his left groin has not improved.

The Yankees say that an MRI administered Tuesday night showed a "small persistent strain of the left groin." The team said he will continue to throw off flat ground but will not throw off a mound for a week.

Pettitte threw about 40 pitches, combined between halfway up the mound and on the mound, during batting practice Tuesday. He then spent about 20 minutes in a whirlpool before going to a hospital for the MRI.

"I'm feeling better," Pettitte said in a statement. "When I reach back 100 percent like I'm trying to get a batter out during a game the groin area grabs."

The Yankees said Pettitte wanted to have the MRI to put his mind at ease.

Pettitte, 11-2 with a 2.88 ERA, has been on the disabled list since July 20. He felt a little stiffness in the groin Friday while throwing 50 pitches in a simulated game in Tampa, Fla.

Also, Alex Rodriguez was out of the Yankees' lineup for their game against the Detroit Tigers because of tightness in his left calf.

"He's still a little sore," said Yankees manager Joe Girardi, who wasn't sure if A-Rod could pinch hit.

Nick Swisher, however, returned to the lineup, saying he will be smarter about taking big swings to protect his sore right elbow.

It's a sharp pain. It's more of a zing when it happens, then it goes away," Swisher said. "These are the dog days right now. This is when the grind starts and I want to be out there."

He hit a two-run single off Justin Verlander in the first inning Tuesday.

Both players left during the Yankees' 3-1 loss Monday to Detroit with the injuries.

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