The veteran right-hander had an MRI on Monday and the test revealed a tear of the ulnar collateral ligament in his right elbow; the date for the surgery is yet to be determined.
Right-hander Winston Abreu was selected from Triple-A Durham on Sunday to take Isringhausen's place on the 25-man roster.
Isringhausen, 36, started the ninth inning against the Nationals on Saturday night. Elijah Dukes greeted him with a single before he retired Willie Harris on a flyout and struck out Austin Kearns swinging. Isringhausen then threw a wild pitch to the next hitter, Corey Patterson, and looked to the dugout, alerting Rays manager Joe Maddon that something was wrong.
Isringhausen had to leave the game and Lance Cormier came on to get the final out in the Rays' 8-3 win.
Isringhausen had season-ending elbow surgery with the Cardinals last season before signing with the Rays at the beginning of Spring Training. He began the season on the disabled list before he joined the team on May 18. He went 0-1 with a 2.25 ERA in nine games with the Tampa Bay this season.
He has compiled 293 career saves, sixth-highest among active players and 22nd on the all-time list. Since 2000, his 284 saves rank tied for third (with Billy Wagner) among Major League pitchers, trailing only Mariano Rivera and Trevor Hoffman. He is also the all-time Cardinals leader with 217 saves.
Isringhausen was selected to the National League All-Star team in 2005 and the American League squad in 2000. He has pitched in five postseasons with St. Louis and Oakland and reached the World Series in '04. In 23 career postseason appearances, he is 4-5 with 11 saves and a 2.36 ERA. Isringhausen saved a career-high 47 games in '04, tied for the NL lead.
Bill Chastain is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.