Indians manager Terry Francona said Masterson will miss some time but hopes Cleveland's No. 1 starter isn't out for the season. An MRI on Tuesday revealed the injury to the right-hander, who made his first All-Star team this season.
Masterson was pulled from Monday's game against Baltimore in the second inning because of soreness in his left side.
Masterson 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. Cleveland is 3½ games out of the second wild-card spot and has lost six of seven.