Pettitte, who turns 41 next month, came out of a start against Seattle on May 16 because of a tight left trapezius muscle, near his pitching shoulder. He went on the DL the following day.
Pettitte said Thursday that the injury essentially was a back spasm. He threw 76 pitches in a simulated game Tuesday in Florida.
"I was pretty locked up back there. If I had to miss a start where we were, with the rotation and stuff like that, it was probably the best thing to do for the team," Pettitte said. "I felt pretty confident that it would just be the two weeks."
Pettitte missed a start in April because of lower back spasms. He's been unable to get into a rhythm between starts.
"That's been frustrating for me just because I have a certain routine, and I want to be on it," he said. "And, obviously, you've got everyone saying you may be doing too much, you might be doing this, you might doing that."
First baseman Mark Teixeira and third baseman Kevin Youkilis are likely to be activated for Friday's series opener against Boston following two-day minor league injury rehabilitation assignments at Double-A Trenton. Teixeira had a partially torn tendon sheath in his right wrist, sustained while taking batting practice with the U.S. at the World Baseball Classic on March 5. Youkilis has not played since April 27 because of a back injury.
"If they're here tomorrow, I'll be happy. If they're here the next day, I'll be happy," manager Joe Girardi said. "So when they get here, they get here."
Girardi said he wasn't sure whether he would keep Robinson Cano batting second when the pair return.
"You want him to have as many at-bats as possible," Girardi said. "That's the bottom line."