Oswalt, 5-3 with a 3.11 ERA, has been diagnosed with a mid-back strain on the left side and is scheduled to return on June 14.
The back-to-back 20 game winner was scratched from his last start on Sunday just two minutes before the game was to begin. At the time the Astros said he had back spasms and Oswalt said it felt like "somebody hit me in the back."
Astros manager Phil Garner said there was a possibility that Oswalt could have been healthy enough to start this weekend, but that he placed him on the disabled list so he could have time to heal.
"He did see some improvement," Garner said. "But I feel
certain that if we go ahead and DL him that we'll have him 100
percent when he's ready to go on the 14th."
Houston brought left-handed pitcher Philip Barzilla up from
Triple-A Round Rock to take Oswalt's roster spot.