Shakur Stevenson Says It'd Be 'Bad Mistake' For Vasyl Lomachenko To Fight Him

Shakur Stevenson says it'd be a baaad mistake for Vasyl Lomachenko to step in the boxing ring with him ... telling TMZ Sports he'll straight up whoop his ass!!

23-year-old Stevenson -- one of the most talented, young boxers in the world -- is fighting Toka Kahn Clary Saturday night (more on that later) ... a fight Shakur says is a warm-up for Loma.

Of course, 32-year-old Loma lost to 23-year-old Teofimo Lopez -- a big-time upset -- in October ... but is still considered one of the best pound-for-pound boxers in the world.

Shakur only has 14 pro fights (14-0, 8 KO's) ... but believes if he ends up in the same ring as Loma, he'll "beat him worst than what Teofimo did."

Why's he so confident about beating a guy who will go down as a legendary fighter??

Shakur thinks Lomachenko lacks heart.

"I don't even think [the Lopez loss was about] power. If I'm gonna be honest with you, I ain't think the power play a part in the fight. It had to do a lot with heart," Stevenson says.

"If we being real, ain't no fighter can win no fight going in for 6 rounds straight and not throwing no punches at all. I think that's heart."

First, Shakur has to beat Clary this weekend, which -- according to Stevenson -- ain't gonna be an issue ... 'cause SS told us there's NO WAY he can lose to the dude.