**WARNING -- VIDEO CONTAINS EXPLICIT LANGUAGE**
Adrian Broner is an OVERRATED LOSER who never should've been compared to Floyd Mayweather ... so says Floyd's uncle/trainer who just blasted the boxer on camera.
After Broner got beat down by Marcos Maidana last weekend, Roger Mayweather trashed the fighter to HustleBoss.com ... saying "[Broner] wasn't what people thought he was."
Of course, Broner was hyped up to be boxing's next superstar -- hype that Roger says was nothing but a crock of BS from the start.
"Who the f*ck did he beat? The next star has to beat somebody."
As for a possible Broner vs. Mayweather matchup, Roger says it ain't ever gonna happen ... because Broner's not capable of beating the talent it takes to get a match with Floyd.
"This motherf**ker wasn't no Floyd. I know god damn well he wasn't."