Rays manager Joe Maddon said after the game that Isringhausen was being looked at by team doctors in the training room.
"He was really starting to throw the ball well, and there's concern," Maddon said. "I don't know what level, but there's concern."
Isringhausen was not available for comment.
The 36-year-old right-hander had surgery to repair a torn flexor tendon in his elbow in September. He was signed in the offseason to a Minor League contract after spending the past seven seasons with St. Louis, where he became the Cardinals' all-time saves leader.
Isringhausen is seven saves shy of 300 for his career. In eight appearances since coming off the disabled list this season, he was 0-1 with a 2.45 ERA.
"It's just really unfortunate because he was throwing the ball really well," said Rays reliever Randy Choate. "He had his curveball back. He looked really good the last couple times out. You feel for a guy like that when that happens."
Isringhausen left the game against Washington with two outs in the ninth and the count 1-2 to batter Austin Kearns. He threw a wild pitch to the back of the backstop and immediately pointed to his elbow as a trainer ran out of the dugout.
Rays reliever Dan Wheeler said he spoke with Isringhausen after the game and Isringhausen told Wheeler he felt something on his last pitch.
"He said it didn't feel too good when he released the ball," Wheeler said. "Obviously I hope he's OK, because I think he's a big part of what we're trying to do here."
With Isringhausen going to the DL, the club will make a corresponding move before Sunday's game.
Zach Schonbrun is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.