Masterson will miss at least one start because of a strained left oblique, another blow to a team struggling to stay in playoff contention. The right-hander was pulled from Monday's game against Baltimore in the second inning because of soreness in his left side and an MRI on Tuesday revealed the injury.
Cleveland manager Terry Francona said Masterson won't resume baseball activities until he's pain-free. An oblique injury takes some players four to six weeks to recover from, which would be too late for Masterson to return.
"I don't know," he said when asked if he could be done for the season. "I'm sure you get worst-case scenario, whatever it is, that could always be the case. We'll take a few days and continue to assess. When it's based off how I feel, it makes it a little more difficult to say for sure what it's going to be. My hope is to make a few more starts by the time the season's over and be ready for the playoffs."
Francona doesn't know if his No. 1 starter will take the mound again in 2013.
"He'll definitely miss a start," Francona said. "After that, you just don't know. He's going to miss a little time here. The hope would be it's not the rest of the season."
Masterson's injury is more bad news for the Indians, who have lost six of seven and fallen 3 1/2 games out of the second AL wild-card spot. Reaching the post-season will be even more difficult without their best pitcher.
"It could take a short amount of time or it could take little bit," Masterson said. "We're hoping for the shortest bit of time."
Masterson, who made his first All-Star team this season, leads the Indians in wins and is tied for first in the AL with three shutouts. The right-hander is 14-10 with a 3.52 ERA in 29 starts. For now, he won't pick up a ball until the pain is gone.
"It's not any worse than it was yesterday," he said. "I've got a touch of soreness over there, maybe a little bit when I cough. Hopefully that will go away in a few days. I'm icing and resting and throwing some prayers out there."
Despite the fact the Indians are playing poorly in their most crucial stretch of the season, Francona isn't going to rush Masterson back.
"We'll go on symptoms," Francona said. "We can't let him start doing anything until he's pain-free. We'll keep monitoring him and see how he's progressing. We're not going to do something that's not in his best interests."
Masterson, listed at 6-foot-6 and 250 pounds, is managing to keep his sense of humour. Asked if doctors found an MRI tube big enough to fit him, he said, "Just barely. We almost went to the zoo."
Corey Kluber, who has been on the disabled list since Aug. 6 with a sprained right middle finger, will start Saturday in Masterson's spot against the New York Mets.