This is A.J. McKee -- one of the hottest young stars in MMA -- and he's REALLY good at submitting people.

So, when he came to the TMZ newsroom the other day, we had him put Lucas in the same crazy armbar he used beat Derek Campos at Bellator 236 in December ... and yea, it was AWESOME!

McKee is undefeated -- 16-0 as a pro -- with the most consecutive wins in Bellator history, including 5 wins by submission (the most in Bellator featherweight history).

Lucas -- who's locked horns with legends like Jon Jones, Brian Ortega, Henry Cejudo, Paige VanZant and Kayla Harrison -- offered himself up as a fight dummy for McKee and learned first hand what it's like to be in his grasp!

As for his fighting future, McKee is currently in the semi-finals of the Bellator Featherweight World Grand Prix ... where the grand prize is a $1,000,000 check!

We spoke with McKee about his career goals, dream fight ... and his favorite moments inside the cage.