Moments after the L.A. Chargers picked Mike Williams in the 1st round of the NFL Draft, he got a phone call from Philip Rivers ... and it's clear both of these guys mean business. 

Williams is pretty excited about his new gig -- and when the former Clemson WR touched down in L.A. on Friday, he told us he vows to be a dominant pro who gives everything to the game. 

In fact, he's so serious about it he won't let himself have an In-N-Out burger because it's not on his diet ... THAT'S HOW YOU KNOW HE MEANS BUSINESS!!