LeBron James says Conor McGregor was "tough as a motherf**ker" during the Mayweather fight -- and joked there's no way in hell he coulda gone 10 rounds with TBE. 

TMZ Sports was right outside the T-Mobile Arena when we spotted Bron and Cavs teammate J.R. Smith breaking down the fight. 

LeBron: "You gotta respect [McGregor]." 

Smith: "The thing is he came out strong in the beginning of every round. He burnt himself out."

LeBron: "You gotta respect him though because he ain't no boxer. Tough as a motherf**ker though."

Smith's retort was hilarious -- "Sh*t, give me $30 million! I'd be tough too for 30 minutes!" 

LeBron: "I'm dropping in the 1st round. I ain't lasting 10."