Isringhausen was activated from the 15-day disabled list on Saturday morning and reliever Mark Worrell was sent down to Triple-A Memphis. The Cards also recalled Anthony Reyes and demoted outfielder Joe Mather.
Scheduled to throw a bullpen session before Saturday's game against Philadelphia, Isringhausen was activated to help out a depleted bullpen following a 20-2 loss on Friday.
"I think after seven years, they've seen me throw enough," Isringhausen said. "That break was good to help me get my mind right."
General manager John Mozeliak said it would have been nice to see Isringhausen throw firsthand, but the reports on the closer from Florida were good enough to convince the club he was ready.
Initially, the Cardinals wanted to call up Isringhausen for the start of their series against the Royals on Tuesday, but had to speed the process up after four relievers threw Friday night.
Because the Cardinals will begin Interleague Play next week, manager Tony La Russa wanted to make the move for a 13th pitcher since they will plug a player into the designated hitter's slot.
"We have a need," La Russa said. "But I think there was a possibility based on him throwing that he would be activated soon. We need him right now."
Instead of reassuming his spot as a closer immediately, the Cards are going to ease him into the position. Isringhausen will pitch the middle to late innings until the club feels he's ready to pitch the ninth.
"Once I start getting people out, I'll be back in the ninth inning," Isringhausen said. "I have no doubts about that."
While rehabbing, Isringhausen said he not only needed to clear his mind, but also work on his mechanics. Cardinals pitchers Chris Carpenter and Josh Kinney both told Isringhausen he was tipping his curveball.
"I just worked on some mechanics and just cleared my head," Isringhausen said. "Now you just kind of start over and put the first month and a half or two months behind me and start from now."
Isringhausen is on the cusp of making history. The Cards' closer is eight saves short of 300 for his career and ranks 22 all-time.
As for Reyes -- who can pitch in long relief -- he's made nine appearances with the Cardinals and posted a 5.27 ERA.
"He's a fresh arm and he's been pitching well down there, so he's a logical choice if we need help," Mozeliak said.
Both Mather and Worrell struggled in their short time with the Cards. Mather, who was called up for Chris Duncan on May 29, hit just .229 with one home run and three RBIs. Worrell made four appearances, giving up five runs in 5 2/3 innings.
"I think this was a great experience for both of them," Mozeliak said. "It allowed them to see what it's all about, and I think they both have to realize they have to make adjustments to play at this level full-time."
Lee Hurwitz is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.