ST. LOUIS (AP) -- St. Louis Cardinals pitcher Chris Carpenter increased the effort in his second bullpen session on Friday, but the 2005 NL Cy Young winner may not return before Sept. 1 when rosters can be expanded.

Carpenter, a 21-game winner in 2005, is eligible to come off the 15-day disabled list from a strained shoulder on Tuesday but said he'll probably need at least two more bullpen sessions before he'll cleared to return. On Sunday, he plans to throwing closer to 100 percent to mimic his normal pre-game routine and has been told he needs to duplicate that effort.

If all goes well Sunday, Carpenter said he'd throw in the bullpen again either Tuesday or Wednesday in what could be his final test.

Carpenter is a key to the Cardinals' wild card hopes, especially after going 0-1 with a 1.88 ERA in his first three starts of the season after coming back from reconstructive elbow surgery that knocked him out after opening day in 2007. But the Cardinals want him to stay in the rotation once they bring him back again.

"He'll start when he's ready," manager Tony La Russa said. "I just know we're going to get him ready as soon but as safe as we can."

Carpenter said he threw 50 pitches besides some long-toss beforehand, and mixed breaking balls into the test.

"We're going to do it again on Sunday, do a little more and a little harder," Carpenter said. "My arm felt fine. It's just a progression of intensity and volume.

"I didn't air it out today."

Carpenter said the Cardinals have not mentioned a rehab assignment and he said he didn't think he'd need one.

(Copyright 2008 by The Associated Press. All Rights Reserved.)