Oswalt pitched just 1 2/3 innings in Houston's 11-6 win before summoning head athletic trainer Nate Lucero and manager Cecil Cooper to the mound. He said the back bothered him slightly in his previous start five days earlier against St. Louis and flared up again in the bullpen two days later.
"I didn't think it was anything major," Oswalt said. "I went out there the first inning and felt OK, not great. I thought I might make it through five or six [innings], but with this lineup, you have to go 100 percent. I started to throw 85, 90 percent to get through, and I said, 'You're going to have to let it go or come out.'"
Oswalt gave up a leadoff single in the first and had retired four in a row before tweaking his back again on a fastball away to Alfonso Soriano with two outs in the second. He threw a curveball for a strike for his 22nd -- and final -- pitch of the game. Oswalt was 4-1 with a 2.40 ERA in his last eight starts prior to Tuesday.
"I can walk but can't do anything strenuous," he said. "Moving around, it's not too bad. I knew I shouldn't stay out there and blow it out. It got to the point I might go out there and throw knuckleballs one or two innings, but that would be pretty tough to do."
Oswalt was on the DL in 2006 with a mid-back sprain and last year with a left hip abductor strain. An MRI of Oswalt's back taken last July showed a small disc protrusion.
Brian McTaggart is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.