SURPRISE, Ariz. -- Rangers reliever Tanner Scheppers has been diagnosed with torn articular cartilage in his left knee and is returning to Texas to have surgery. The initial diagnosis is he will be out until after the All-Star break.

Scheppers was bothered by a bone bruise in his left knee last season, but the Rangers believe this is an unrelated injury. Scheppers had the knee scanned in September and the results were clean.

"He woke up a few days ago and the knee was a little puffy," Rangers general manager Jon Daniels said. "He can't think of a moment where he tweaked it. It's obviously a blow, a disappointment for him. But we'll do what we have to do to get him back."

The knee is the latest in a series of setbacks for Scheppers. He was 6-2 with a 1.88 ERA in 76 games as a setup reliever in 2013. But he pitched in just eight games in 2014 while dealing with right elbow inflammation, and dealt with the knee and a sprained right ankle last year, as he compiled a 4-1 record with a 5.63 ERA.

He went into the offseason eligible for arbitration and the Rangers showed their confidence in him by tendering a contract, despite his multiple issues. They were also openly pleased with his progress in offseason workouts at Globe Life Park.

"All winter long I saw him at the ballpark, and he had energy and a great look on his face," manager Jeff Banister said. "To not get rewarded for that is extremely disappointing and disheartening to him. But there is a competitor inside Tanner that will find a way to push through this. He'll come back and find a way to be competitive."