UFC's Roy Nelson Says Van Damme Coulda Fought UFC

Jean-Claude Van Damme doesn't just talk the talk when it comes to MMA fighting -- he's a REAL athlete who coulda competed in the UFC back in his prime ... so says Roy "Big Country" Nelson.

Roy isn't just an accomplished UFC fighter in his own right -- but spent some quality time with JCVD on the set of the upcoming "Kickboxer: Vengeance" flick.

And with Van Damme recently lecturing UFC fighters about how weak their leg strikes have become -- we had to ask Roy if he thinks JC could back up his tough talk.

For the record, Van Damme is an accomplished martial artist -- who won several fights as a teenager before becoming an action star.

There's more ... we also asked Roy how he'd fare in a showdown with Hafthor Bjornsson -- the MASSIVE guy who plays The Mountain on "Game of Thrones."

The 400-pound Bjornsson famously had a sparring session against Conor McGregor -- but Nelson says he'd get CRUSHED against someone his own size.