Chicago White Sox pitcher Jake Peavy will be out 4-to-6 weeks with a non-displaced rib fracture on his left side.
Peavy had an MRI prior to Wednesday's game against the Seattle Mariners, which revealed the break.
He exited Tuesday's game early against Seattle after allowing a two-run double to Raul Ibanez with one out in the third inning. Following a brief conference with manager Robin Ventura and the team trainer, the former Cy Young Award winner exited the game having thrown just 42 pitches.
Prior to exiting, Peavy surrendered six runs for the second straight time out, during which his ERA rose from 2.97 to 4.30. He threw a complete game against the Marlins prior to the two poor outings.
Ventura said that left-hander Hector Santiago will take his place in the rotation. Santiago, who threw one-third of an inning in relief Tuesday, is 1-4 with a 3.40 ERA in 15 games (five starts) this season.
No other roster moves have been announced.