Masterson, 28, showed some discomfort after throwing a pitch and was visited by the trainer and manager Terry Francona. He remained in the game, threw a few more pitches, then showed more discomfort before being lifted.
In 28 starts coming into Monday, Masterson pitched to a 3.49 ERA (108 ERA+) and 1.21 WHIP in 188 1/3 innings. He's been very durable since moving into the rotation full time, throwing at least 180 innings in each of the last four seasons.
The Indians have not announced any kind of timetable for Masterson following the rib injury.