Houston (51-49) at Chicago (52-46)

The Houston Astros have managed to stay in the thick of the NL Central race despite an injury to slugger Lance Berkman(notes). They now might have to press on without their staff ace.

A day after Roy Oswalt(notes) left with an injury, the Astros look to win their second straight Wednesday against the Chicago Cubs at Wrigley Field.

In Tuesday’s 11-6 victory over the Cubs (52-46), Oswalt pitched a scoreless first inning, then struck out the first two batters he faced in the second, but could only throw two more pitches. Oswalt, who was 4-1 with a 2.40 ERA in his previous eight starts, left with a strained lower back.

“I know I shouldn’t blow it all the way out. It was not to the point that I could have thrown knuckleballs to make it through one or two innings,” said Oswalt, who is flying to Houston on Wednesday and was unsure if he will be getting an MRI on his back.

Houston, which moved within 2 1/2 games of NL Central-leading St. Louis with Tuesday’s victory, is already without Berkman, who was placed on the 15-day disabled list Thursday with a strained left calf muscle.

“To lose him (Oswalt) at this point of the year is not a good idea. Everybody is crossing their fingers that he will be OK in a couple of days,” said Miguel Tejada(notes), who hit a two-run homer and had a season-high five RBIs.

Just as Houston’s offense has needed to step up with Berkman going down, the Astros are going to need a strong outing from scheduled starter Mike Hampton(notes) (6-7, 4.74 ERA) after the bullpen has worked 12 1-3 innings in the first two games of this series.

Hampton hasn’t been up to the task lately, though.

He has allowed 14 runs in 15 innings in his last three starts after going 3-1 with a 1.97 ERA in his previous five. In Friday’s 5-4 win over the New York Mets, the left-hander gave up four runs, eight hits and four walks with seven strikeouts in 5 2-3 innings. Hampton was able to help himself, though, with a two-run homer off Mets ace Johan Santana(notes).

Against the Cubs on May 6, Hampton allowed five runs - three earned - and seven hits in 5 1-3 innings of a 6-3 loss. He is 4-0 with a 4.46 ERA in his last six starts at Wrigley, but hasn’t pitched there since 2004.

Houston’s bullpen could be further depleted Wednesday without Wesley Wright(notes), who was rushed to the hospital following Tuesday’s win because he was experiencing discomfort in the area around his appendix.

Chicago had a five-game winning streak end Tuesday, as it dropped a half-game behind the Cardinals.

Despite the loss, Aramis Ramirez(notes) continued his power surge.

Ramirez hit a three-run homer and is 13 for 26 with three home runs and 10 RBIs in his last six games after batting .190 (8 for 42) with one home run and two RBIs in his first 11 games since coming off the DL with a dislocated left shoulder

Scheduled starter Randy Wells(notes) (6-4, 3.10) has been benefiting from Ramirez and the offense, which is averaging 6.5 runs in the last six games.

After going 0-3 despite a 2.55 ERA in his first seven starts, Wells has won six of his last seven outings with the offense backing him with at least four runs in each victory. In Friday’s 8-5 win over Cincinnati, the right-hander allowed three runs and seven hits in six innings.

A bullpen meltdown kept Wells from earning the win in the Cubs’ 5-4 victory over Houston on May 16. He allowed four hits in six shutout innings before the Cubs yielded four runs in the top of the ninth.