Jason Hammel will have arthroscopic surgery on his right knee and is expected to be out for at least a month. The Orioles right-hander was placed on the disabled list on Sunday. "I can't keep pitching on a bad leg," he said. "I start feeling things with my arm and hurting stuff there. I'm obviously bummed out." Hammel's knee has been bothering him since May, but the injury became more serious Friday night when he was forced to leave a game against Detroit after throwing a pitch in the fourth inning.