Greg Hardy Roughin' Up UFC Fighters at Practice, Says Boxing Coach

Greg Hardy made a damn good decision switching over to MMA ... 'cause the former NFL star is already takin' out UFC fighters -- so says his boxing coach, Billy Padden.

Billy wouldn't name names ... but told us Hardy's killin' it at American Top Team (ATT) -- an MMA gym stacked with heavy hitters like Junior dos Santos and "Bigfoot" Silva.

"He actually knocked 2 UFC fighters down in sparring," Padden told TMZ Sports.

Billy says Greg's a "natural born fighter" -- and even has a shot to be 1 of the best in the world.

So when it comes to the opponent for his first amateur fight -- Joe "The Legend" Hawkins -- Padden told us The Kraken's gonna dominate.

Billy also defended Hardy from the haters ... explaining how he's making the most of his 2nd chance.