Michaels, 47, is currently in critical condition. "After several CAT scans, MRIs and an angiogram, [doctors] decided to keep Michaels in the ICU and are running several tests to determine the cause. [It] will be touch and go for the next few days while he is under intense observation," the source says.
Reps confirm this report is accurate, and tell PEOPLE, "We will have no additional information until further testing is done."
The rocker's latest hospital visit follows his emergency appendectomy in San Antonio, Texas, on April 12. There is no word yet whether this new development is related to the rocker's recent appendectomy or to his diabetes.
After his appendectomy, Michaels remained in the hospital under his doctors' care – then transferred to a rehab facility specializing in diabetic patients.
As he was recovering, Michaels – who was still in the running to win this season of NBC's Celebrity Apprentice – seemed to be in good spirits and updated fans on his progress.
"They told me that if I had gone on stage like I wanted to, [my appendix] likely would have ruptured and I could have died," he wrote in a note to his fans. "There is just no way around the fact that getting your appendix out HURTS. I have a pretty good threshold for pain, but this one hurts."