"The doctors, I didn't tell them I was going to do this. I never told them I was coming here. They're going to find out in a few minutes!" Michaels' toldUs Weekly at Wednesday night's American Idol finale.
"I may not have told them, either," Michaels' added when asked whether he had told his family.
"To be honest, there's still obviously a lot of pain I'm still experiencing," Michaels told Us. "[But] I feel great."
The former Poison frontman performed "Every Rose Has Its Thorns" with American Idol third-place finisher Casey James, and he said he was approached to appear on the finale before he suffered his various medical problems.
"I needed to do this tonight, just to come here It's my first musical performance back and after this, I'm hoping to go out on the road, and I'm really looking forward to it," Michaels told Us before claiming his heart was "pumping" and describing how it felt to perform.
"I got on stage, first of all, the fans and friends and family were all here. It was such an awesome feeling to see everyone screaming and cheering and singing along with the song. I was like, 'Hey, that's three generations out there.' That's all I've wanted to do is have a great music career and to be able to do that, 20-some years later, that's a great feeling."
Michaels' younger sister told Us that just by watching her brother, she could tell "his body is not where his spirit is" -- which the former Rock of Love star agreed with.
"Here's the thing. They have not said everything's okay. I'm running about 75 percent realistically, physically," he explained to Us. "But in my soul and my heart, I feel great."
Michaels said he still plans to perform this weekend in Biloxi, MS to gage if he's ready to completely return to the stage.
"I'm going to see how that goes and just take it a few shows at a time," he told Us.
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American Idol ninth-season runner-up Crystal Bowersox described Michaels' finale performance as "inspiring."
"With his health issues and the things that he's been through recently, it's inspiring to me that he gets up and pushes through it," she told Us.