Oswalt pitched 6 1/3 innings and gave up four hits and two runs while tying a season high with eight strikeouts before his back got the best of him. He collected his 15th no-decision, which ties an Astros single-season record, but the Astros went on to a 5-4 win.
"We thought it would be best to be careful and not take any chances," Houston manager Cecil Cooper said of pulling Oswalt. "He didn't get the win, but he pitched real well -- really good, as a matter of fact -- probably as good as last time out, considering the lineup he had to face this time."
Oswalt, who was shutting out the Phillies on two hits through six innings, left the game with head athletic trainer Nate Lucero after allowing two singles with one out in the seventh. Oswalt missed two weeks earlier this season with a left lower back strain, but he said this injury doesn't appear to be as severe.
"It started getting tighter and tighter, and I went out there in the seventh and it never really loosened up," Oswalt said. "I thought it would be smart to get out of there now without making it go all the way out and maybe missing a start. Hopefully I got out of there in time and I can get back out there."
Oswalt suffered a back injury on July 28 at Wrigley Field and missed two weeks before returning Aug. 11. He has pitched at least six innings in his past five starts, going 2-1. Oswalt is scheduled to face Atlanta on Thursday, and the Braves are the only National League team he's never beaten in the regular season (0-2 in six starts).
Brian McTaggart is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.