Too many ifs there. Both these guys had problems with giving it 100% yet you allow Douglas to step out of his own skin and fight a faultless fight, while still presenting him with a version of Tyson beset with problems. That is a bit like having a computer chess progran designed to always produce a win for one side. In practice I suspect that Douglas would have slipped further and been less motivated. The logical outcome would follow.
Hard to say Douglas was in such great shape and so motivated after the death of his mother. Would the fire be there again. Would the first win make him over confident or will he take it and build off it and fight a greater fight. Tyson had alot going against him in 1990 divorce, shitty corner, involved with Don King, getting away from his great training and fighting methods and becoming more of a head hunter than the great boxer he was. Would Tyson learn from his loss and come in more motivated and more serious. He could not have taken Buster all that seriously. Hell he even nearly KO'd him. A better prepared Tyson could have found home with a big punch like the first fight and could have stopped it. However after Douglas Tyson clearly was never the same, he surrounded himself with the wrong people and peaked before his time. His style went from a great fast boxer puncher quick moving head and body attack with great orchestrated combinations to a one punch slugger. Result, unless Buster is way out of shape and terribly motivated and did not train he does the exact same thing as the first fight.