I have to think this would be just about the biggest upset I've ever seen if McGregor won, so I'm going with Mayweather.

The few examples we have in the past of fighters dabbling in other styles suggest that they generally get their ass kicked. Even Anderson Silva had to throw in the towel (literally) when he boxed a rather average boxer a back in the day, and I really don't know any MMA fighters who were craftier than him. Of course, he'd later win a boxing match, but this is basically the greatest MMA fighter ever and he was certainly not fighting anyone close to Mayweather.

Mcgregor

Mayweather.

Mcgregor, like any fighter that steps into a ring has a punchers chance, but that's it.

Mayweather will dance and weave, and continually hit Mcgregor with body shots. Most of Mcgregors shots will hit air, and he will be repeatedly punished for it.

I think a better question is will Mayweather finish Mcgregor?

As a fan of mma, I'm selfishly worried a potential savage beating hurts Conor's mma career. Conor gets tagged in most of his fights, and although Floyd isn't renowned for his punching power or finishing ability, these blows accrue, and Floyd will likely tag him repeatedly. Several rounds of pops to the head could mess him up, even without a finish. I've seen brutal fights alter the trajectory of a fighters carreer many times.

I've trained in boxing a tad and in Mixed Martial Arts for about fourteen years, and I don't think McGregor stands much of a chance. I would like if he won, since I find Mayweather's domestic violence history rather off-putting, but there is a huge difference in their talent levels when it comes to the actual bout they are engaging within - that of boxing. Certainly, MMA fighters use boxing as a part of their repertoire, but Floyd is on a different level, the guy has trained since he was 6 or 7 and is the best boxer of his generation. Conor has never even fought in a single amateur fight. It's going to be a lopsided beating, unfortunately. 

Of course, if this was a kickboxing or an MMA match (or even just the two guys getting into a fight in the street), I would take McGregor all day every day. Grappling is incredibly difficult for those without any experience in it. 

He doesn't have a power advantage that's a misconception. He has no strength behind his punches as he doesn't use his body for strength. He almost exclusively throws arm punches. He doesn't have a good center of balance. In short he throws a punch like a UFC fighter but that doesn't work with actual gloves on. Also as far as stamina even though younger I definitely give the advantage to mayweather. He has been fighting 12 round boxing matches for 20 years and stamina has always been a strength of his. Connor will be extremely winded come round 8 or 9 If he is still standing.

btw if u think conno punches hard YouTube his sparring sessions. Then watch maywethers. One is a seasoned vet and the other is a clear amateur who may have less experience boxing than I do.

I edit the predictions in about an hour.

is it 6hours from now?

In regards to Roach his assessment is dependent on what he was initially expecting he may not have had high expectations to begin with in that regard.