Machine Gun Kelly's Drummer Robbed and Hit by Car, Hospitalized

Machine Gun Kelly's drummer got attacked, robbed and hit by a car ... according to law enforcement sources, but he says he's going to bounce back.

We're told JP Cappelletty -- better known as Rook, MGK's drummer -- was walking down a street in the Hollywood Hills Tuesday night around 8 PM, when 2 guys jumped out of a vehicle to rob him.

Our law enforcement sources say there was a physical altercation, and Rook had thousands of dollars worth of property stolen from him. We're told at some point, he was also hit by a car, which injured his foot and arm.

Rook posted a pic from his hospital bed Wednesday showing his right arm and hand in a cast, and another with MGK visiting him. Rook says, "Don't Worry I'll Bounce Back."

Our sources say the suspects got away, and while it's unclear if they were armed ... they certainly used force to rob the drummer.