Oswalt plans to rejoin the Astros in Atlanta on Sunday in time to accompany the team to Pittsburgh, where he will throw on flat ground on Monday, followed by a bullpen session on Tuesday.
Oswalt, who's nursing a left hip abductor strain, was throwing off flat ground on Friday, and the righty felt pain in a slightly different spot than where he originally experienced it when he left his start in Houston last Monday.
"When he swings his hip out, it was bothering him," Astros general manager Ed Wade said. "[On Friday], he said the discomfort was a little higher. Rather than to say it must be linked to something, we just sent him back to Houston to check it out. No sense for [head athletic trainer] Dave Labossiere to have to be in a position to try to figure that out."
"He tried to put more into his throws and it didn't work," Astros manager Cecil Cooper said. "They'll check him out again, and whatever the doctor recommends is what we'll do."
The Astros' rotation for the Pirates series will be Runelvys Hernandez on Monday, Backe on Tuesday and Brian Moehler on Wednesday.
Alyson Footer is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.