Cooper decided to push Oswalt's next start back three days. Instead of taking the mound Saturday in Atlanta, Oswalt will start Tuesday in Pittsburgh. If all goes well, Oswalt will also pitch Sunday, July 13, in Washington, the last game before the All-Star break.
Chris Sampson will take Oswalt's place on Saturday as a spot starter.
Cooper is taking the precautionary route with Oswalt, who is continuing to recover from a left hip abductor strain. Oswalt will still receive two starts before the All-Star break, so there really is no difference between starting Saturday and Thursday, or Tuesday and Sunday.
"You want to get him as many [starts] as you can, but the main thing is health," Cooper said. "We have to get him to 100 percent. That's the bottom line.
"I don't want him to go out there at 90 percent. He's too big of an investment. He's too big of an asset to us."
Oswalt, who threw on flat ground Thursday morning and will likely throw a bullpen session Sunday in Atlanta, was fine with the decision.
"Saturday would be pushing it a little bit," he said. "I don't want to completely miss a start, because then you've got 10 days where you don't throw. Hopefully, I'll start Tuesday and the last day before the break."
Sampson started the year in the rotation, but was moved to the bullpen on May 30 in deference to Brian Moehler, who was having more success in a starting role. Sampson has posted a 2.16 ERA over 13 relief appearances. As a starter, he was 3-3 with a 6.04 ERA over 10 starts.
Sampson realizes he is probably receiving a temporary promotion, but is happy to return to his former role, even if only for one game.
"Whenever [Cooper] gives me the ball, I'm going to do the best I can," he said. "I really haven't been stretched out since I've been in the bullpen -- I don't think I've pitched two innings in, I would guess, over a month. I just told Coop I'll go as hard as I can for as long as I can."
Alyson Footer is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.