Oswalt (7-8, 4.56 ERA) underwent an MRI exam on Saturday that confirmed a strained left hip abductor and also revealed the disk problem. But manager Cecil Cooper said Oswalt should be ready to start again on July 28 for the opener of a three-game series with Cincinnati.
"It just got to the point where he couldn't really throw off a mound," Cooper said. "We're going to try to get him on the mound a couple of times before his start."
The Astros will recall Runelvys Hernandez from Triple-A Round Rock to fill Oswalt's roster spot. Hernandez, 0-3 in three starts for Houston this season, will start Monday's game against Pittsburgh.
Oswalt injured his hip in the sixth inning of his June 30 start against Los Angeles. He re-aggravated the injury in his next start on July 11, leaving the game after throwing only 17 pitches against Washington.
Oswalt felt sore again after a light workout on Friday. He was scheduled to throw a bullpen session on Saturday, but had the MRI exam instead.