DeSean Jackson -- Jadeveon Clowney Needs to Prove Himself ... ON THE FIELD!

He's big, he's strong and he's fast as hell ... but it doesn't mean JACK SQUAT until Jadeveon Clowney proves himself on Sundays ... so says DeSean Jackson.

The Philadelphia Eagles speedster was leaving Supperclub in L.A. last night -- when we asked if he was impressed with Clowney's blazing fast 40-yard dash time at the NFL combine.

"Faster than me? I don't think so," Jackson said.

"I can just congratulate him for being 260 and doing his thing ... but this Jackson speed right here is rollin' on anything in the NFL ... put it that way, you feel me?"

We feel you.

There's more ... "I just need to see him get to the NFL and do his thing ... the NFL is a big man sport."

"I know he's 260 ... that fast sh*t cool ... but I got homies right here that's bigger than that, that poundin' sh*t in the streets. So, if he gonna get on that field and pound sh*t, I can't do nothing but respect him."

*Note -- we know our guy totally botched Jadeveon's name ... sorry.