Prime Alexander Karelin in today's HW division. How would he do?

I know that the idea of him fighting in MMA excites you (it does sound exciting, I wish we could somehow see him fight in MMA), but we can't be that confident that he would be an unbeatable force. There are so many aspects in this sport and remember that although Karelin was as dominant as anyone, he was never trained to defend submissions, only grappling moves.

To be fair, this doesn't prove that he has a good chin. My guess is that he had a great chin, but we can never know.