Oswalt left the game after one inning with a left hip abductor strain, the same injury that sidelined him for two starts prior to Friday's outing.
The right-hander said he felt "100 percent" warming up in the bullpen before the game, but he felt some discomfort after throwing a pitch right before the national anthem. He threw a few more pitches, but the pain didn't subside.
"I thought maybe I could go out there and try to put up three or four [innings], at least," Oswalt said.
Houston manager Cecil Cooper was leaning toward not allowing Oswalt to make the start, but Oswalt was adamant, and Cooper finally relented.
Seventeen pitches later, Oswalt was out of the game. He allowed one hit and a walk in the first frame and was replaced by Chad Paronto, who hit for Oswalt and then took over on the mound for the next 2 1/3 innings.
"You could see he got through the first inning, but he wasn't Roy the first inning," Cooper said. "He kind of held back a little bit. His stride was a little shorter than normal. He might have hit 92 or 93 [mph] a couple times, but he was pretty much at 89. We decided just to not let him continue, because we can't take a chance and injure him further than that."
This start was Oswalt's first since June 30. Last Saturday, he flew to Houston from Atlanta to receive a pain-killing shot. Upon returning to the team on Monday, he said he felt great and was ready to get back on the mound.
Oswalt received the shot near the sciatic nerve in his lower back, which had become inflamed and was causing the pain in his left hip. The sciatic nerve is the longest and widest single nerve in the body, and an irritation can cause pain in the lower back and legs.
"It's stimulated from my back some, but I think from my hip hurting, my back's trying to take over and take a little load off my hip," Oswalt said.
He was pain-free during a bullpen session on Wednesday, which makes Friday's setback so surprising.
"I threw a bullpen two days ago, three days ago and was throwing as hard as I could throw and felt great," he said. "Today, warming up, I didn't feel anything. I felt 100 percent. I came back just a little too quick, I guess. Maybe it wasn't all the way healed. The way I felt, I felt great."
Oswalt's next scheduled start is next Friday against the Cubs, the first game after the All-Star break. However, he could start the second half on the disabled list, although that will be determined at a later time.
"We'll just wait and see what the doctors say," Cooper said.
Alyson Footer is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.