Mercer could stop him, but I'd give the edge to Tubbs by decision. That's the likely outcome, but that doesn't mean it would be the outcome.
I could see Tony putting on a clinic. He has to be on his game though. Mercer had real power, but could be out boxed obviously. Someone mentioned the Damiani fight, Tubbs at his best is slicker and faster than Damiani. So he could win this by boxing SMART.. Damiani got too aggressive late in his fight with Mercer and got caught.
Larry Holmes, regarding the heavies of the 90's compared to the 80's... "But the heavyweights out there were not getting better; they were getting worse. Tyson doesn't do the things he used to, and Razor Ruddock, he's a one-arm fighter. I give myself a shot. But I've also given myself a deadline." https://www.nytimes.com/1991/04/07/sports/boxing-holmes-explains-comeback.html