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 to The Beast.