D'Angelo Russell's impromptu Lionel Messi impression just set him back a wad of dough -- the Lakers guard was just fined $15k for punting the rock into the stands after Saturday's double-overtime win over the Warriors.
The NBA just handed down the punishment on Monday ... with the league's executive VP Joe Dumars announcing the costly penalty.
The moment happened after the final buzzer of the second extra period ... when Stephen Curry heaved the ball three quarters of the court in a last-ditch effort to win the game.