Think about this. Seriously... Say Mayweather enters MMA at the lightweight division and were to take on some battle tested guys in that division. These are all potential opponents he would face off the top of my head....
BJ Penn vs Mayweather
Norifumi Yamamoto vs Mayweather
Takanori Gomi vs Mayweather
Sean Sherk vs Mayweather
Vitor Ribeiro vs Mayweather
Frankie Edgar vs Mayweather
Augusto Frota vs Mayweather
I cannot realistically see Flyod winning any of these bouts under MMA rules... Sure he has a "puncher's chance" but do you REALLY think he can defend from being taken down and then survive down there? How about dealing with knees from the clinch?? He will have to disappear for about 2 years and train strictly wrestling, BJJ & some Muay Thai and THEN this can be a serious discussion. He certainly is a world class athlete so he can probably do decently well if he respects all the disciplines and trains in them all for an extended period. But within a year from now? Forget it!