"I'm fine. It just cramped up on me," Lester said after the Red Sox earned a 4-3 win in the series opener. "It got a little worse as the game went on."
Lester reached for his hamstring after his third pitch of the inning. He called for the trainer and was removed from the game in favor of right-hander Vicente Padilla.
Lester appeared to slip on the mound when he threw his 96th pitch. He walked off the field under his own power, but with a noticeable limp.
The lefty started to cramp up in the fifth inning, but remained in the game. Lester made it through three more frames before the pain proved to be too much.
"He started feeling it a couple innings before he let himself come out," catcher Ryan Lavarnway said. "He was out there fighting and he wanted to be on the mound. He probably shouldn't have been out there for the eighth anyway. He was having trouble even straightening out his leg. He wanted to be on the mound, and his stuff was so good it was hard to take him out."
Lester allowed three runs on six hits over seven-plus innings. He struck out six against four walks. His eighth victory of the season was his first at Fenway Park since June 27 against the Blue Jays.
"It was a great pitching performance," manager Bobby Valentine said. "He had all of his pitches working. There weren't a lot of hard-hit balls."
Lavarnway said it was the sharpest he's seen Lester pitch all season.
"His cutter was overpowering guys, his curveball was slowing them down, and then he was locating his fastball," the catcher said. "That was the best that his stuff has been and he pitched like he meant it, and that was great to see."
Austin Laymance is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.