So massive, that Real Madrid brought UFC star Khabib Nurmagomedov to meet him in Spain so the 2 could bro down ... this according to Khabib himself.

"He know everything about the UFC," Nurmagomedov told TMZ Sports outside Jay Glazer's Unbreakable Performance Center in Hollywood.

FYI, the meet-up went down at Real's training center in Madrid ... where the two Euro-sport mega-stars talked fighting and exchanged gifts (Khabib got a signed ball and jersey).

If this feels familiar, it's probably 'cause Conor McGregor -- Khabib's arch-nemesis -- also kicked it with Cristiano. Ronaldo visited Conor before his first Nate Diaz fight ... facing off with McGregor at his private gym in Vegas.

No way of knowing which fighter Cristiano likes more -- Ali and Khabib claim it's Khabib, of course -- but Ronaldo did look pretty pumped wearing Khabib's signature papakha hat.