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03/29/11 11:22 PM ET

Southpaw Happ suffers right oblique strain

Astros starting pitcher J.A. Happ left Tuesday's exhibition game against their Triple-A affiliate in Oklahoma City after three innings with a strained right oblique.

"I'm taking an optimistic approach and we'll see how I feel tomorrow," Happ said. "Hopefully, I got out before anything too serious [happened]. But it tightened up pretty good. I hope I caught it before it gets too serious."

Happ, who allowed two runs and four hits, is scheduled to start Sunday's game against the Phillies in Philadelphia. He's supposed to face Roy Oswalt, the pitcher he was traded for last July. He went 5-4 with a 3.75 ERA in 13 starts for the Astros last year.

"I know how frustrating [oblique strains] can be," Happ said. "That said, if you don't take care of them right away, they linger. We'll know more when I wake up tomorrow."

The Astros have been hit hard by injuries during Spring Training. They lost starting catcher Jason Castro likely for the entire season with a torn anterior cruciate ligament in his knee in early March, and last week, starting shortstop Clint Barmes broke a bone in his hand when he was hit by a pitch. He's out four-to-six weeks.

Brian McTaggart is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.