Ashton Kutcher -- Real Deal Jiu-Jitsu Ass Kicker ... Says Legendary Trainer

Ashton Kutcher's first move after wrapping "Two and a Half Men" could be becoming a world champion in Brazilian Jiu-Jitsu, if he wants it -- no joke -- according to his famous trainer.

Rigan Machado tells TMZ Sports he heard all the haters after Ashton earned his purple belt earlier this year. Detractors thought it was just a case of Hollywood favoritism, but Machado says Kutcher is a serious terror on the mats.

Rigan says Kutcher is the best celeb he's ever trained -- which doesn't seem like a high bar -- but he adds ... Ashton could beat some pros ... "because his background is in [high school] wrestling he learns so fast."

Machado has a big Jiu-Jitsu event scheduled for January ... and says he's invited Kutcher "to come and compete if he wants to, if he decides to compete he will do will good."