The A's made the move Saturday. They also optioned right-handed pitcher Josh Lucas to Triple-A Nashville and recalled righties Chris Bassitt and Carlos Ramirez from their top affiliate.
Casilla experienced a similar injury in spring training, A's manager Bob Melvin said. The injury isn't thought to be serious.
Casilla is 0-0 with one save and a 3.32 ERA in 16 games. He helped San Francisco win three World Series championships.
The 37-year-old has complied 144 saves in 15 seasons. He had 16 saves as the A's closer for parts of last season.
"Obviously he has a little different workload this year," Melvin said. "He's been prepared for it and he's been great for us, but he felt it enough last night to where we feel like we need to give him some rest."