St. Louis ? Until a few days ago, Chris Carpenter had been eagerly anticipating his return to the St. Louis Cardinals’ rotation. Now the staff ace’s next start might be in August.

The team announced Saturday that Carpenter, sidelined since starting opening day by an elbow injury, will undergo arthroscopic surgery to trim bone spurs that have caused persistent swelling. He’s expected to be out at least three months after the team’s medical staff conceded that a rehab plan that called for rest followed by a gradual pitch buildup wasn’t working.

“It got to a point where it’s either do it or continue to battle this,” Carpenter said after St. Louis’ 13-0 loss to the Astros. “We need to get it taken care of so the rest of the season’s not in jeopardy.

“Hopefully we can get it done and my recovery will be faster than everybody hopes and I’ll be back faster than everybody hopes.”

Dr. George Paletta, the team physician and medical director, scheduled the surgery for Tuesday. Barring further setbacks, he anticipated that Carpenter would pitch again this season.

The plan calls for Carpenter to resume throwing six weeks after the operation, and Paletta estimated he’d need six weeks to get his arm in shape.

“Anytime we operate on the elbow there is risk,” Paletta said. “I think the likelihood is very good but there are no guarantees. Absolutely not.”

Carpenter was a 15-game winner last year and the NL Cy Young award winner in 2005. The loss is another blow to a team that won the World Series last fall but is now coping with the death of reliever Josh Hancock and a scuffling start that has landed them in last place in the NL Central.

The Cardinals have lost six of their last seven games, getting outscored 51-10 in the process, and their 11-17 record is the franchise’s worst start since an identical record in 1990.

“Obviously, you can’t replace a guy like that,” infielder Aaron Miles said. “It’s a tough loss and he’s going to be missed big-time. We’ve just got to grind it out.”

Brad Thompson, a middle reliever with the Cardinals who made 15 minor league starts in 2004 and got one spot start with St. Louis last May, will fill Carpenter’s slot in the rotation on Tuesday.