Athletics manager Bob Melvin said the club is not certain how the injury occurred. It's possible the injury happened when Chavez was trying to brace himself before being hit in the hip by a batted ball Sunday against the Texas Rangers.
Chavez was scheduled to pitch Wednesday against the Chicago White Sox. He was 7-15 this season with a 4.18 ERA. Cody Martin, who pitched last on Sept. 7 at Triple-A Nashville, will start in Chavez's place.