Astros ace Roy Oswalt will skip his next scheduled start because he still felt pain in his lower back after a bullpen session Monday.

Forced to leave his previous outing because of an irritated disk, Oswalt was scheduled to start Tuesday night against San Francisco. After throwing 30 to 40 pitches in the bullpen, he wasn't ready to go.

Oswalt said he would meet with doctors on Tuesday and throw another bullpen session on Wednesday.

"I'm not 100 percent," Oswalt said. "I just don't feel good."

Oswalt (6-4, 3.61 ERA) left in the second inning of Houston's 11-6 win in Chicago on July 28.

Houston manager Cecil Cooper said right-hander Felipe Paulino will be recalled from Triple-A Round Rock to start Tuesday's game. The team will announce a corresponding move after Monday's game.

Oswalt is due to start again on Saturday against Milwaukee. General manager Ed Wade left open the possibility that Oswalt could go on the disabled list, retroactive to July 28.

"We leave everything available," Wade said. "We'll see how he feels on Wednesday. If he's continuing to signal that he's having some discomfort, then we'll huddle up at that point and have a serious discussion about the potential of doing that."

The Astros hope slugger Lance Berkman and left-hander Wandy Rodriguez return this weekend.

Berkman has been on the disabled list since July 23 with a left calf strain.

"I can definitely still feel the injury, but it's definitely not as sharp as it was," Berkman said.

Rodriguez (10-6, 2.63 ERA) left Saturday's loss in St. Louis with a strained right hamstring. He threw on flat ground and jogged before Monday's game. Depending on Oswalt, Rodriguez could start again on Saturday or Sunday.

"Wandy, I'm hoping he can take his turn," Cooper said. "We have to protect these two guys. That's our 1 and our 2 (starters)."

The Astros also announced that backup outfielder Darin Erstad will play for the team's Double-A affiliate in Corpus Christi on Tuesday and Wednesday. Erstad has been on the disabled list since July 19 with a left hamstring strain.