Not counting actual professional athletes who have been in movies — no Ray Allen in He Got Game, etc. Same with Hustle.

Been too long since I've seen Finding Forrester — were the guys in that convincing?

Same with Love and Basketball.

Guys in Pain & Gain were convincing, if weightlifting counts.

I was convinced by both brothers in Warrior, though I don't know MMA that well. I know Tom Hardy competes in Tae Kwon Do or something.