Oswalt (7-8) struck out nine and allowed one run in six innings of Houston's 4-1 win. He grounded out in the bottom of the inning and then appeared fine when he returned to the mound for the seventh.
After a few warmup pitches to backup catcher Humberto Quintero, assistant athletic trainer Rex Jones and manager Cecil Cooper came out to check on Oswalt. The pitcher left the field with Jones.
Oswalt said he was a sore driving home from the game but said he felt better than expected on Tuesday. He didn't think the injury was serious enough to land him on the disabled list.
"I think I'll be OK," he said. "Just going to see how it feels in two days."
Oswalt is due to start again Saturday in Atlanta.
"If I do miss, maybe it's only one or two more days," said Oswalt. "Either way I look at it, I'm going to get two starts before the All-Star break."
Cooper said he would wait until Thursday or Friday make a final decision on Oswalt's availability for Saturday's game. He said he would consider calling up Jack Cassel from Triple-A Round Rock or moving Chris Sampson from the bullpen to make the start if Oswalt isn't ready.
"We're not going to cross that bridge until later in the week," Cooper said. "We'll think about some of those things, but we'll wait and see how it is."