Geoff Blum hit a tiebreaking homer in the seventh inning and Tejada connected and had a season-high five RBIs, helping the Astros overcome an early exit for Oswalt because of a strained back to beat Chicago 11-6 on Tuesday night.
Jeff Keppinger had four hits and drove in three runs as the Astros snapped the Cubs five-game winning streak and ended their own three-game losing streak.
"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 Tejada.
Oswalt pitched a scoreless first inning then struck out the first two batters he faced in the second. After throwing a 1-1 pitch to Alfonso Soriano, catcher Ivan Rodriguez noticed something was wrong with Oswalt and met with him on the mound. He then waved for Astros manager Cecil Cooper and a team trainer. Oswalt has a strained lower back.
Moments later, Cooper called left-handed reliever Wesley Wright to replace Oswalt, who left with the Astros leading 1-0.
Oswalt said he felt pain in his back in the last inning he worked in his last start against the Cardinals on July 22. He then experienced pain in a bullpen session two days later, but still felt he could make Tuesday' start against the Cubs.
"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. Oswalt was unsure if he will be getting an MRI on his back.
Oswalt had won his three previous decisions and was 4-1 with a 2.40 ERA in his last eight starts.
After the game, Wright was rushed to Northwestern Hospital by ambulance after experiencing discomfort in an area around his appendix.
Aramis Ramirez hit a three-run homer for the Cubs, which dropped a half game behind St. Louis in the NL Central.
Blum broke a 6-all tie in the seventh with a two-out solo home run off Angel Guzman (2-2). The Astros added on in the eighth inning. With runners on first and second, Keppinger hit a two-run triple to center off Cubs reliever Aaron Heilman. Then Tejada followed with an RBI single.
Astros reliever Jeff Fulchino (4-3) pitched three innings to earn the win. He allowed three runs on three hits.
"Fulchino stepped up big time for us and you can just go down the list," Cooper said. "Keppinger and Tejada definitely teamed up. We just gutted it out. This is probably the best win of the year for us."
Cubs starter Ryan Dempster, who was activated from the disabled list before the game, ran into trouble in the third inning. Wright led off with his first career hit, a single to left, Bourn followed with another single and Keppinger had with an RBI single.
Dempster made a throwing error as the Astros scored another run. Carlos Lee had a sacrifice fly to make it 4-0.
Dempster said he felt fine in the bullpen session before the game, but the layoff between his last start on July 2 clearly showed.
"It looked like it. I felt good going into the game. I felt ready. Just didn't do a very good job," Dempster said.
The first three hitters in the Astros lineup, Bourn, Keppinger and Tejada were a combined 9-for-14 with eight RBIs and eight runs scored.
"In front of our 3, 4, 5 hitters if we can get on base we're going to be a tough team to reckon with," Keppinger said.
In the fifth inning with the Cubs trailing 6-3, Kosuke Fukudome hit a bouncer back at Fulchino. The ball bounced inside Fulchino's jersey. He wasn't able to find it in time and Fukudome reached safely. With one out, Fulchino walked Lee, then Ramirez hit a three-run shot to left. It was his eighth of the season.
Dempster allowed six runs -- five earned -- on nine hits in five innings. He struck out three and did not allow a walk.
Before the game, the Astros placed reliever LaTroy Hawkins on the 15-day disabled list because of shingles in his back. ... After the game, Houston recalled RHP Bud Norris from Triple-A Round Rock. ... To make room for Dempster, the Cubs optioned RHP Justin Berg to Triple-A Iowa.