Hasim Rahman Jr. isn't buying the hype surrounding Jake Paul ... telling TMZ Sports the Problem Child is not the hard-hitter people think he is -- calling him a "whole b****."
"A lot of people expect the right hand of Jake Paul to be this devastating punch," Hasim says. "I try to tell people he can't punch, but don't listen to me."
Remember, Hasim and Jake were supposed to cut the trash talk and fight for real on Aug. 6 ... but the bout was canceled due to a weight issue (although Rahman believes Jake purposely scrapped it out of fear).