The veteran right-hander had season-ending elbow surgery in 2008 while with the St. Louis Cardinals, signed with the Rays in spring training and started this season on the disabled list.
He left a relief appearance against the Washington Nationals Saturday after throwing just 2/3 of a inning and Sunday went back on the DL.
Manager Joe Maddon said on the team's Web site the pitcher would undergo magnetic resonance imaging Monday.
""It's an elbow strain," Maddon said. "We won't really know until we get the MRI. I just know he didn't feel real good."
Isringhausen, a National League All-Star in 2005, is 0-1 with a 2.25 earned run average in nine relief appearances for the Rays this season. He has 293 career saves, sixth-highest among active major league pitchers, but none this season.
Copyright 2009 by United Press International.