Oswalt, who looked sharp by holding the Pirates to three hits and one run while striking out six batters in four innings, said he wasn't concerned about the hamstring. Astros general manager Ed Wade said Oswalt felt some discomfort in his left glute while throwing in the batting cage Sunday.
"On Sunday, he started the game and the rain came, and he went into the cage to finish off his throwing, and a couple of pitches into it he felt a pinch of some kind in his glute muscle, his rear end," Wade said. "So they've been treating that, and he was cleared to go today. At some point today, he felt something on the same side with this hamstring. With the back history and so on and so forth, it makes sense to pop him in there and get him looked at."
Astros manager Brad Mills said he was concerned about the timing of the injury.
"This late in camp, there's always concern, no doubt about it," he said. "The one positive on that note is he wanted to go back out. He said he felt the strain in his left hamstring. He's concerned about it, but he's not so scared that he wasn't willing to go out there and pitch."
Oswalt threw 60 pitches Friday in his longest start of the spring. He threw four scoreless innings March 16 against the Red Sox, but had his start Sunday against the Mets rained out after he pitched one scoreless inning, forcing him to throw more pitches in the batting cage.
"I felt a little tight in the cage, but nothing major," Oswalt said. "Today, I didn't feel anything. My hammy got tight toward the end. I told them I could go a couple of more innings, but it's Spring Training. It's nothing major. If I needed to pitch seven [innings] today, I could have gotten through it."
Oswalt, who last week was named Houston's Opening Day starter for the eighth consecutive season, has a faulty disk in his lower back that has caused him problems the past few years. He finished last season on the disabled list with a lower back strain.
"When you throw in the cage and throw on the side, it's nothing like throwing in the game," Oswalt said. "You don't get the same response from the hitters, so you don't know what you've got that day. You could throw good pitches, but it's kind of like going to the driving range and hitting some balls. You don't know what you get until you get on the course. Overall, going from 60-something pitches from 40 [on Sunday], I'm in good shape."
Oswalt is the second consecutive Astros starting pitcher to leave a game with a minor injury. Right-hander Brett Myers left Thursday's game against the Phillies in Clearwater with a mild left-groin strain. Wade doesn't expect Myers to miss a start.
Brian McTaggart is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.