A brutal knee injury clearly hasn't derailed Mark Zuckerberg's MMA career ... 'cause the Meta honcho was just seen mixing it up on the mats with none other than UFC champ Alex Pereira!
Check out some vids the light heavyweight strap holder posted on his Instagram page on Thursday ... he and Zuck really got after one another in an intense training session.
Pereira posed for a pic with Zuck afterward .