EXCLUSIVE

Jean-Claude Van Damme has a brand new 'Kickboxer﻿' costar -- UFC fighter Roy "Big Country" Nelson ... and we're told the heavyweight CRUSHED IT on the set.

Our sources tell us ... "Big Country" was added to the cast after Paige VanZant dropped out due to a training conflict and to prepare for her upcoming fight ... and they were blown away by Nelson's acting skills.

In fact, we're told director Dimitri Logothetis initially only had Nelson slated for a minor scene in Nevada ... but he was so good, producers expanded his role ... and he's currently filming a scene in Thailand!

We're told Nelson plays a prisoner who's training for some kind of death match. He joins a cast stacked with real life athletes including Mike Tyson, Georges St-Pierre, Fabricio Werdum and Ronaldinho.

By the way, Nelson fought in Vegas on July 7th -- and was hit so hard in the face he lost a tooth!!!

But hey, that's what the prop department's for!