To those hating on Bronny James, John Salley's got a message for y'all -- pipe down!!!

The former Lakers center defended LeBron James' son and L.A.'s decision to pick him up in June's NBA draft this week ... saying straight up, all of the teenager's detractors are simply "mad they didn't think of it or do it" themselves.

"Listen," the four-time NBA champion told TMZ Sports out at LAX. "If my dad had the power, if my dad was the leading scorer, if my dad was able to take a pay cut so I could get a check -- it's like no other business."

Bronny, of course, was a star in high school ... earning McDonald's All-American honors following a helluva career at Sierra Canyon. But, when he joined USC, his career never quite took off, in part due to the cardiac arrest he suffered in a preseason workout.

Still, the Lakers took him with the 55th overall pick -- and haters everywhere screamed nepotism.

Salley clearly thinks all those whining should shut it -- telling us he and everyone else would do the same thing if they were in the James Gang's shoes.

In fact, Salley's advising LeBron to stay at it and play in the league a few more years ... so he can hoop with his other son, Bryce!