CHICAGO (AP) _ St. Louis Cardinals reliever Mike Timlin had arthroscopic surgery Friday to repair torn knee cartilage and is expected to miss 4-to-6 weeks.

Timlin injured his left knee while stretching and squatting in the bullpen during the fifth inning of Thursday's game against the Chicago Cubs at Wrigley Field.

Timlin flew back to St. Louis and Dr. George Paltetta, the head of the Cardinals' medical staff, operated on a torn medial meniscus at Barnes West County Hospital.

``It is definitely an unusual circumstance,'' Cardinals trainer Barry Weinberg said Friday. ``It's just a simple thing he's done for years. Why? I don't know.''

Timlin should recover in time to pitch again this season.

``We expect him to,'' Cardinals manager Tony La Russa said.

Timlin is 3-5 with a 3.44 ERA this season and has been used primarily as a setup man.

``I don't know what happened. It just locked up on me,'' Timlin said after Thursday's game, adding he'd never had a knee problem.

``We're losing a very key guy out of the bullpen, but you couldn't call us short,'' La Russa said. ``We've got guys who need the work.''