Usain Bolt would get his ass kicked if he lined up against John Ross in the 40-yard dash ... so says Ross' college teammate and Packers draft pick Kevin King.

TMZ Sports talked to King -- a CB selected by Green Bay with the 1st pick of the 2nd round -- outside The Logan Hotel in Philly the night he was drafted.

We asked, who wins in the 40? Bolt, 9x gold medalist, and fastest man to ever walk on Earth, or Ross, who's 4.22 40 yard dash is the fastest ever recorded at the NFL Combine.

King's answer, John Ross, "no doubt."

We also talked to King about his new teammate Aaron Rodger's golf date with model/actress Kelly Rohrback