ATLANTA --   As Jordan Walden proved quite efficient through this season's first month, he managed to hide the fact his left hamstring had been bothering him dating back to Spring Training. But the discomfort the right-handed reliever felt during last week's appearance against the Giants let him know it was time to take a break.

After getting a chance to evaluate the ailment for a few days, the Braves announced Saturday afternoon that Walden had been placed on the 15-day disabled list, retroactive to May 5. Left-handed reliever Ian Thomas was recalled from Triple-A Gwinnett to take Walden's roster spot.

 "I was praying," Walden said. "I went out there and gave it a shot a couple days. I threw a bullpen [session] and it just wasn't happening."

After allowing the Mets two runs April 9, Walden limited opponents to four hits in the 9 1/3 scoreless innings during his next appearances. His dominant run ended last Sunday when he allowed Giants shortstop Brandon Crawford to cap a two-homer day with a two-run shot in the eighth inning.

"I threw one more pitch and thank God I got out of the inning," Walden said. "That one kind of told me, 'It's had enough and I need to get it better.' I just need to make sure I get that better before I hurt my arm."

Walden will rehab at the Braves Spring Training facility this week. Though he will be eligible for activation May 20, it is too early to know whether he will need more than two weeks of rest.

David Carpenter, Luis Avilan and Anthony Varvaro will likely serve as Atlanta's primary setup men until Walden returns.

Walden's violent delivery, during which he essentially vaults himself toward the plate off his left leg, may force him to miss more time than he hopes.

After he strained both groin muscles in late August last year, he returned three weeks later and allowed at least two runs in three of his final four regular-season appearances.

"Every day I get better," Walden said. "So hopefully, it just takes me 15 days. The hammy will tell me when it's ready."  

When the Braves sent Thomas to Gwinnett on Sunday, they were planning for him to have a chance to work on his secondary pitches while serving as a starting pitcher. But the 26-year-old southpaw's only start ended up being the one he made Tuesday when he tossed three scoreless innings against Buffalo.

Thomas estimated that nearly a quarter of the 50 pitches he threw during Tuesday's start were sliders. As he made the first 10 appearances of his career this year with the Braves, Thomas' secondary pitch selection consisted of a curveball. But last week, Braves pitching coach Roger McDowell suggested the reliever should begin toying with the slider.