Jimmy Fallon's so stoked on Giancarlo Stanton joining the Yankees, dude went straight PRIMAL when we asked him about the blockbuster trade.
"OOOOOOOOOH ... what a season we're gonna have, oh my goodness!" Jimmy grunted/said to TMZ Sports in the Big Apple.
Fallon's New York born and bred ... so he couldn't even try to play it cool talking about Stanton -- and new manager, Aaron Boone -- putting on the pinstripes.
The real question -- how are Jimmy and Giancarlo gonna top Aaron Judge's appearance on "The Tonight Show?"