Oswalt left Friday's game against the Pirates after four innings as a precaution with tightness in his left hamstring, a few days after feeling tightness in his left gluteus muscle. Astros team medical director Dr. David Lintner and head athletic trainer Nate Lucero determined the discomfort stemmed from Oswalt's chronic lower back issues and sent him to see Dr. Michael McCann.
Houston general manager Ed Wade was pleased with the news and said the shot was a "maintenance issue."
"That was good," Wade said. "There was a purpose in sending him to Dr. McCann, because Nate and Dr. Lintner thought what he was experiencing was emanating from the back, and Dr. McCann gave him an injection and said he's good to go Wednesday and is on time to make his start for the opener, so it's all good."
Oswalt, 32, is scheduled to pitch Wednesday in a Minor League game, manager Brad Mills said. The righty threw 60 pitches in four sharp innings against the Pirates in his most recent spring start, but he has had two other starts washed out by rain. Oswalt has allowed one earned run in 10 innings this spring.
Mills was relieved upon hearing the update on his ace.
"It's the same thing he felt when he left [for Houston], that he was able to come back and be fine," Mills said. "His last outing, he wanted to throw another inning or two. It's positive, all positive."
The health of Oswalt and slugger Lance Berkman are huge keys for the Astros this year. Berkman has been out for more than two weeks after undergoing arthroscopic knee surgery March 13, and the team faded from contention in July when he went down last year with a calf strain.
Wade said Monday the Astros could be forced to put Berkman on the disabled list in the coming days, which would mean he would miss at least the first four games of the season. Mills said Berkman participated in drills Monday in Kissimmee, Fla., and is progressing.
"If he's not at the point of getting into games in the next couple of days, then I think the prudent thing would be to go ahead and disable him and give him time to get ready," Wade said. "We're not there yet. He could come in tomorrow and say, 'I'm good to go on Wednesday.' We could leave him back here and he could play in [Minor League] games all the way through Sunday if it looked as if he were moving in a straight line and would be ready for Opening Day."
Oswalt has made at least 30 starts in every season since 2004, and his elbow and shoulder have remained healthy. He had offseason surgery on his groin following the 2003 season, and he was on the disabled list in 2008 with a hip abductor strain.
The three-time All-Star has been dealing with a bulging disk in his back since '07. The disk is pushing against a nerve that descends down Oswalt's left leg, and last year was causing hip pain and loss of feeling in the leg. McCann recommended Oswalt try to strengthen his core last year to help his back, because surgery wasn't a viable option.
Mills hopes the back won't be an issue again.
"We're not going to have expectations that way, but hopefully because of all the work he was able to do in the offseason with his back to keep it strong, that he won't feel this all year long," the manager said. "But at the same time, it is a possibility."
The Astros shut Oswalt down in the middle of September last year as a precaution, but likely would have kept him on the field had the team not been out of the playoff race. He still managed to make 30 starts and was 8-6 with a 4.12 ERA.
"Dr. McCann was very pleased with his alignment and thought he was symmetrical," Wade said. "According to Nate, it was all good news. The shot, I would term as a layman as basically a maintenance issue. Our expectation is this will resolve whatever was going on.
"I hope he makes 35 starts without ever uttering Dr. McCann's name again, but we'll handle things when they come, if they come."
Brian McTaggart is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.