WWE's Paul Heyman: Brock Lesnar Will Kick Francis Ngannou's Ass, Just Like Roman Reigns

Anyone dumb enough to call out Brock Lesnar -- like UFC star Francis Ngannou -- is gonna end up with a 1-way ticket to the ER ... so says Brock's advocate.

We got Paul Heyman in Dallas after 'Monday Night Raw' ... just hours after Lesnar laid a VICIOUS beating on WWE superstar Roman Reigns -- which got Reigns carted off on a stretcher.

Heyman told us that's the fate that awaits Ngannou, and any UFC and WWE heavyweight that steps up to The Beast.

"It's one thing to ask for a fight with Brock Lesnar. It’s a whole different story when you actually have to FIGHT Brock Lesnar. Just watch the footage," Paul told TMZ Sports.

We did. It's nasty. That said, we've also seen Francis nearly decapitate a man inside the Octagon.