Jean-Claude Van Damme has a brand new 'Kickboxerufeff' 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 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 .