TMZ has obtained a recording of a jailhouse phone call between Murray and a friend ... in which the Doc says he was rushed to USC Medical Center earlier this week after fearing he was suffering from deadly blood clots in his legs.

"It progressed until the leg felt numb ... it was painful all to the top of my foot ... around the medial aspect of my ankle and all the way to the mid-calf."

Murray -- who was a licensed doctor before he killed Michael Jackson -- explained, "Blood clots are relatively risky ... especially if they propagate from the legs into the lungs to cause pulmonary embolism which can result in permanent disability or even death."

Ultimately, doctors who are currently licensed determined there were no blood clots and returned Murray to his cell ... where he says he continues to suffer.

"I have had to lay in bed for most of the day because there is no way to elevate my feet. One of the recommendations by the doctor is to keep my legs elevated and to increase my activity which is impossible in a 5x7 cell."

During the call, Murray gripes that his feet have become so swollen, "I may never be able to wear a boot again."

Murray then issues a veiled threat -- "I don't want to be permanently hurt by any condition in here ... because if I am, I will not let it rest."