Bob Costas -- Disses 50 Cent ... Seriously

Bob Costas' street cred is at an all-time high ... 'cause the guy just talked some serious smack about 50 Cent.

Check it out -- we were talking to Bob about the fact he got a "do-over" after a terrible first pitch at a St. Louis Cardinals game earlier this week -- which Bob says he earned because the 1st pitch didn't accurately reflect his ability.

And that's when Bob took the dig at Fiddy ... explaining why the rapper didn't deserve the same mulligan he got when he botched a 1st pitch at a Mets game back in May.

"50 Cent couldn't have done better because HE CAN'T THROW A BASEBALL. It was gonna go sideways AGAIN."

Bob continued, "I knew I could do better because I had done better my whole life. And then I did."

Congrats Bob ... you may have just started a rap war.