"What you never know is how a guy is going to respond," Matheny said. "When you take somebody as athletic as Joe, you increase your odds a little bit [of a quicker return]. I think history tells you with that injury, where it's located, it could be a little longer. We're not going to rush it. I think people have seen what happens there. But we also don't think it's a prolonged period too much beyond what 15 days would be."
This is the first time that Kelly, who entered Thursday as the National League's ERA leader with a 0.59 mark, has ever been sidelined due to injury at any professional level. In his discussion with Matheny on Thursday, Kelly, who returned to St. Louis to be examined, described himself as "sore."
The Cardinals made a wave of transactions Thursday that will reshape their pitching staff while Kelly is sidelined.
The club optioned right-handed reliever Keith Butler to Triple-A Memphis, and rookie right-handers Eric Fornataro and Jorge Rondon were slated to arrive from Memphis in time for Thursday's series opener against the Nationals.
Butler had appeared in two games for St. Louis.
Fornataro and Rondon will both be in the Majors for the first time. Fornataro, a former sixth-round Draft pick, appeared in four games for Triple-A Memphis this season and did not allow a run. Rondon, who was originally signed by St. Louis out of Venezuela, led Memphis with 51 relief appearances last year.
Both are 26 years old. Fornataro has a career 25-43 record and a 4.00 ERA in seven Minor League seasons, and he had a 6.02 ERA in 37 appearances for Memphis last season. Rondon is 16-43 with a 4.86 ERA in his Minor League career, and he's exclusively been a reliever since 2011.
Fornataro will wear uniform No. 67 and Rondon will wear No. 68.
Jenifer Langosch is a reporter for MLB.com. Read her blog, By Gosh, It's Langosch, and follow her on Twitter @LangoschMLB. This story was not subject to the approval of Major League Baseball or its clubs.