You must have missed the Tyson / Buster Douglas fight. Douglas beat Tyson's ass by out-boxing him (and by not crapping his shorts before the first round). Ali, whatever else you might say about him, could flat-out box. And he could take a punch - go ask George Foreman about that.
Tyson would have been toast.
Tyson was in his prime 4 years before the Douglas fight. Like I said, his prime was very short lived. Everyone has an opinion. Mine is that Cassius Clay would have had a come to Jesus moment had he fought a 20 year old Mike Tyson. It might have even saved Clay from becoming a lousy mooslim.