You need a jab to have any chance and beating Floyd and not just a flea flicker but a world class jab and Ray had one.
You need to be able to keep up with Floyd at the very least in the speed department and Ray was probably faster then he is.
Combination punching. Floyd has barely had a punch landed flush on him even in the fights he struggled in like Castillo 1 or Oscar. Ray would land on him and often. I dont think anyone has ever come close to landing a 2 or 3 punch combination on Floyd. Leonard would and what would the effects of that one certainty be?
Activity levels. Its one thing to keep some of the guys he's fought at bay he has fought by being tricky and working behind that shoulder, turning them with your feet and countering from some check angles and throwing a punch here and a punch there.
Quite another to be able to pull that off with a guy like Ray with his matching speed, punch output and selection along with crisp 3 and 4 punch combinations.

In most cases its Floyds opponents the end up looking like a deer in the headlights but I think the roles would be reversed in this hypothetical.

And just for ones viewing pleasure