Cincinnati (40-46) at Houston (44-43)

Roy Oswalt hasn’t pitched since starting the Houston Astros’ historic six-man no-hitter nearly a month ago.

The slumping Astros will be happy to have him back on the mound as they open a four-game series with the visiting Cincinnati Reds.

Oswalt suffered a strained right groin after tossing a hitless inning against the New York Yankees on June 11. Five Houston relievers combined to complete the no-hitter and Oswalt wound up on the disabled list the next day.

The Astros’ ace has a mediocre 4-4 record this season, but boasts a solid 3.03 ERA to go with 71 strikeouts in 12 starts.

Oswalt hopes to pad his record against the Reds. The 25-year-old right-hander, who was rehabbing at Triple-A New Orleans, is 7-0 with a 1.57 ERA lifetime against Cincinnati.

Houston is looking to rebound after suffering a three-game sweep to the Pittsburgh Pirates over the weekend. The Astros managed just four hits off Pirates starter Jeff D’Amico in Sunday’s 8-3 loss.

The Reds have lost 11 of their last 15, falling six games below .500 (40-46) for the first time since April 29.

“We’re just playing bad baseball right now, plain and simple,” said Aaron Boone. “As simple as it sounds, that’s it. But there’s no time to pout about it. It’s time to do something. Obviously, that was a tough one today.”

Cincinnati closer Scott Williamson blew a save for the fourth time in 24 chances as the Mets rallied to beat the Reds 7-5 on Sunday.

“It’s the midpoint of the season,” Williamson said. “We’ve got to get going.”

STANDINGS: Reds - 4th place, 5 1/2 GB, NL Central. Astros - tied for 2nd place, 2 GB, NL Central.

REDS PROBABLE STARTER: Ryan Dempster (2-5, 6.65 ERA). Dempster has not won since beating Tampa Bay 2-1 on June 12. He’s 0-1 with a 5.48 ERA in five starts since. The 26-year-old right-hander gave up seven runs and nine hits over four innings as the Reds suffered an 8-7 loss to Pittsburgh on Thursday.

ASTROS PROBABLE STARTER: Oswalt (4-4, 3.05 ERA). Oswalt has faced the Reds 10 times during his career, including eight starts. He held them to one run and four hits over eight innings on April 8, but did not receive a decision in Houston’s 2-1 loss.

OFFENSIVE LEADERS: Reds - Sean Casey, .299 avg.; Adam Dunn, 24 HRs and 54 runs; Austin Kearns, 58 RBIs; Boone, 12 SBs. Astros - Jeff Kent, .313 avg.; Lance Berkman, Morgan Ensberg and Jeff Bagwell, 15 HRs; Berkman, 53 RBIs and 56 runs; Adam Everett, 6 SBs.

SEASON SERIES: Astros, 2-1.

STREAKS AND NOTES: Reds - RHP Luke Prokopec will have more shoulder surgery Tuesday, sidelining him for the season. He had surgery to repair a torn labrum last Sept. 18, and has been on the disabled list since the Reds got him from Los Angeles in the winter meeting draft. … Thirty-eight of OF Dunn’s 47 RBIs have come off homers. … 3B Boone was selected to his first All-Star game as a reserve Sunday. Boone will be the Reds’ lone representative at the game in Chicago on July 15. Astros - OF Berkman is batting .355 with 14 home runs in 45 career games against Cincinnati. … LHP Billy Wagner will be Houston’s lone representative in the All-Star game. He has 22 saves in 25 chances. … The Astros have been swept three times in the last month: June 13-15 at Boston, June 23-25 at Arizona and the Pittsburgh series.

ROAD/HOME RECORDS: Reds - 19-24 on the road; Astros - 26-17 at home.

INJURIES: Reds - OF Wily Mo Pena (hamstring), 15-day DL; LHP Kent Mercker (back), 15-day DL; LHP Gabe White (groin), 15-day DL; RHP Prokopec (elbow), 60-day DL. Astros - 2B Kent (shoulder), 15-day DL; IF Jose Vizcaino (wrist), 15-day DL; LHP Carlos Hernandez (shoulder), 15-day DL; RHP Rodrigo Rosario (shoulder), 15-day DL; RHP Brian Moehler (elbow), 60-day DL.