Martin Perez suffered a broken ulna in his left forearm when he was struck by a line drive during the sixth inning of Sunday's loss to the Mariners.
Perez is a left-handed pitching prospect.
He is not expected to resume a throwing program for approximately four weeks while the fracture heals.
"I didn't see where it hit him. He was on the ground and I just ran out to see him," Ron Washington said. "He got hit pretty hard."