I don't think this is absolute as everyone on here is predicting. In 57 pro fights Michael Moorer was only stopped by Foreman, Tua and Holyfield. I believe 91' Tyson loses to all three of those fighters as well which he clearly did to Holyfield. Moorer beat a version of Holyfield that had only lost to Bowe at that point while Tyson was cleanly beaten once by him and quit in the rematch. I think 91' Tyson would unquestionably be the favorite and I would likely put my money on him catching Moorer at somepoint, but if the best version of Moorer could get him past the mid rounds, I could definitely seeing him frustrating the hell out of and beating Tyson with that right jab and combination punching. Again, we are talking about the 91' version of Tyson not the 87'/88' guy. Don't forgot how Tyson quit in multiple fights (Holyfield II, Williams, McBride) once they got past a few rounds where Moorer never quit against anyone and got up from multiple knockdowns throughout his career. Moorer didn't have a great chin but it is was not as bad as a lot if folks make it out to be. Again, 57 fights and only Knocked out by Foreman and Tua and stopped by Holyfield after getting up from multiple knockdowns.
I think we're underrating Michael Moorer these days. That said, I think in 1991 Tyson catches him at some point.
No way Moorer lasts the distance against 91 Tyson. It is far more likely that fight looks like Tyson-Stewart than it does like Moorer-Holyfield I.
Lets keep in mind that Moorer tend to get up more often after beeing dropped. How many rounds does he make against Tyson in two meetings?