J.A. Happ was unable to start for the Philadelphia Phillies on Monday because of a strained a side muscle. He was replaced on the mound by Jamie Moyer.

The team said Happ hurt himself while taking batting practice Saturday.

"I felt something and it hasn't gone away," Happ said Monday before the Phillies played the Houston Astros and tried to avoid a four-game sweep.

Moyer (12-9, 5.07) started 22 games this season before he was moved to the bullpen Aug. 10. The 46-year-old left-hander pitched three innings in relief Friday, giving up a run on one hit, a solo homer by Carlos Lee.

Happ (10-4, 2.77 ERA) expects to make his next start this weekend.

"I've had this before and it's one of those things where they have a way of being unpredictable and lingering," he said. "We're trying to take the right approach here and knock it out before it gets worse."

Happ was wearing a wrap of ice before Monday's game.

"This wasn't easy to come to this conclusion, but I think it's the smart thing for sure," he said. "I'd be really frustrated if I did try to go today and suddenly, it's six or seven starts instead of one or two. It's just a frustrating situation."