Well, we have an excample on film of this: “Jackass.”
In a sequence mid film, they pulla prank where Johnny Knoxville and pro boxer Eric “Butterbean” Esch parade into a department store in full boxing regalia, complete with a girl holding the round number and stuff, and they commence fighting.
Knoxville is visibly frightened and just covers up; Esch knocks him down just hitting his gloves, but a glancing blow to the head leaves Knoxville stunned on the ground. Very, very unwisely, Knoxville gets up slowly. Esch offers to let him take a swing for free and drops his gloves. Knoxville, who is no wimp - he’s a bit over six feet and, at the time of the movie, was in fine shape - does so, and it has no effect. Esch swings twice, hitting the gloves once and Knoxville’s head once, and Knoxville goes down like his batteries just cut out. The total time they actually spend fighting? Sixteen seconds.
Afterwards Knoxville is shown being taken to the hospital, and no one is laughing. The entire movie is a bunch of stunt men doing ridiculous things, mostly to hurt themselves, and a lot of the things they do you think “Jesus, that guy could broken his neck” - but in the entire movie, no one gets hurt nearly as badly as Johnny Knoxville does in that sequence. Not even close. Minutes after the fight ends, with a paramedic standing over him, he still doesn’t look like he knows where he is, and it’s the only part of the movie where you can tell the cast is legitimately worried about the aftermath of the stunt.
And Esch wasn’t even trying his hardest. The difference between professional athletes and schmucks is just light years.
If you gave me a year to train and get to my ideal weight - well, first of all, I’m a heavyweight (even when perfectly fit) so I’ll be matched up again guys who can do incredible damage with their punches. I guess the question is, how much money would you have to pay me to fight a top level heavyweight boxer, on the assumption I cannot refuse the answer the first bell or deliberately slip and fall before the guy can hit me? It would have to be life changing money, I’ll tell you that, because the guy could hurt me really badly.