"They said it's a strained ligament, a strained tendon and a strained capsule," Isringhausen said.
The right-hander underwent an MRI exam in Kansas City on Friday. However, the Cardinals are waiting on their own head team physician, Dr. George Paletta, to review the results before they decide whether to make a roster move.
"There's clearly some reason for concern at this point," general manager John Mozeliak said.
Isringhausen felt something when he was trying to stretch between innings on Thursday. He retired Gary Sheffield to end the sixth, and as he was trying to stretch out his hips, he evidently knocked something in his knee out of place. He pitched two more innings, however, and, in fact, turned in some of his best pitching of the year after sustaining the injury.
"I'm not a doctor, but to me, I'm like, if I was able to pitch two innings on it, then it's a pain issue," Isringhausen said. "If they can get rid of the pain, I can pitch. Now, they might say if I go out there and try to pitch on it, it could tear it. I don't know. I'm going to talk to the doctor about it."
Matthew Leach is a reporter for MLB.com.
This story was not subject to the approval of Major League Baseball or its clubs.