LeBron James, Lakers Lose It After Austin Reaves Nails Game Winning 3

L.A. Lakers guard Austin Reaves came up BIG for his squad last night -- the rookie hit a game-winning 3-pointer in overtime -- and the locker room celebration was WILD!

The Lakers were in Dallas taking on the Mavericks in a back-and-forth game that came down to a last-second 3-pointer by LeBron James ... that he missed.

So, cue OT, and insert Reaves -- the undrafted former Wichita State and Oklahoma star -- who, as it turns out, has ice water in his veins .... knocking down a 3-pointer off a dime of a pass by Russell Westbrook, propelling the Lake show to a 107-104 victory.

After the game, the celebration moved from the court to the locker room ... where Lakers vet  DeAndre Jordan poured a bucket of ice-cold water on Reaves' head -- as Bron tossed him a game ball.