Ohhh, I see a battering and poor Rocky s getting it. Old Holmes fought and beat Mercer with good old fashioned counter, defensive trickery. Marciano would trade and go to war, but he'd be suffering middle aged pains and immobility s. Mercer trades to and belts Rocky s face in to a bloody mess. Mercer wins by 7 rounds stoppage.
Not sure this question makes much sense considering Holmes despite being over 40 was extremely active coming off 5 straight fights going into the Mercer fight while Marciano had been retired for 9 straight years. Marciano wouldn’t jump in the ring with a top contender coming off an almost 10 year layoff. George Foreman mentioned how foolhardy Holmes was to jump in the ring with Tyson coming off a two year layoff. Even Big George himself coming off a ten year layoff fought 20 plus fights against lesser competition before challenging a champion and he didn’t let any rush him.
You think a 46 years old man wearing a toupee and ketchup for blood in a fake fight is a credible opponent? Don't let us detain you further!
If we're assuming that Rocky has the benefit of modern hair restoration methods, Marciano with a full hair transplant would wreak havoc on the early 90s heavies.