Ryan Garcia's win over Devin Haney was an instant classic, and boxing great Shawn Porter is already salivating over talks of a rematch ... telling TMZ Sports he thinks the Dream can get revenge the second time around!!

Garcia (24-1) beat Haney (31-1) and shocked a ton of fight fans by winning via majority decision on Saturday ... making it the latter's first professional loss of his career.

Porter gave us some insight into what Haney should do next ... and in the immediate future, he believes it's best the pugilist take a vacation and ignore the critics on social media.

"Do everything you can do to get away from this," Porter said. "Decompress!"

But, after some R&R -- it's time to train for some revenge.

"When you're ready, come back to me and we'll talk about it and we're scheduling you for a rematch with Ryan Garcia at whatever weight you want that rematch at, at whatever date you want that date to be."

Porter -- a former two-time welterweight champ -- also knows how Haney can win, too ... re-adjust his "offense to defense" speed to protect himself from the counter move.

"He's hesitant because he knows that that defense isn't there after the offense. So, I think that's something they'll work on in preparation for a rematch," Porter said. "That adjustment kinda is all he needs because he's got everything else."

"Devin has a humongous opportunity to beat Ryan in the rematch."

Of course, Garcia vs. Haney II might not happen ASAP ... Ryan called out Isaac "Pitbull" Cruz, who currently holds the WBA super lightweight belt, after the win.

Plus, fans want Garcia to have another go with Gervonta "Tank" Davis ... or take a fight with Terence Crawford or Shakur Stevenson.

Haney also has options while he waits on King Ry ... including Adrien Broner, who made a fight offer on Sunday.