DON'T DO IT -- unless you wanna embarrass yourself.

"Guys coming from boxing -- stay there," Chuck told TMZ Sports ... when we spoke to the UFC legend about Andre Berto trying to take his talents inside the Octagon.

The Iceman ain't throwin' shade at Berto ... he's just keepin' it real on guys pulling a reverse Conor McGregor ... cause it's A LOT harder to go from boxing to MMA.

If Conor fought Floyd Mayweather in a non-boxing match, for example ...

"Floyd wouldn't have made it out of 1 round -- not kickboxing, not MMA, not a street fight -- not 1 round," Liddell said.

Chuck did have an important piece of advice for boxers trying to make the switch ... but he still thinks it's a bad idea.

Sorry, Andre.