By TMZ Sports

Manny Pacquiao's promoter tells TMZ Sports that a rematch with Floyd Mayweather is "more likely than not" ... but says there are still some serious obstacles to overcome.

Both fighters have now retired since their mega-fight last May. The fight generated over half a billion dollars and 4.6 million pay-per-view purchase. Mayweather retired last September after beating Andre Berto at the MGM Grand in Las Vegas. Pacquiao retired at the same venue this past Saturday night when he decisioned Tim Bradley.

We spoke with Top Rank's CEO Bob Arum who says, "It's close to 50/50 but probably leaning more toward the fight happening ... more likely than not."

Arum says he hasn't spoken with Manny or Floyd specifically about the rematch -- but says, "If the match was available, I believe [Manny] would come back for that match."

Arum also says he knows Showtime, who work very closely with Mayweather, have serious interest in a Pacquiao rematch -- and that's an important component to making the fight happen. Their pay-per-view last May was handled by a joint partnership between rival networks Showtime and HBO.

So, what are the obstacles? For starters, both guys claim they're retired ... with Manny focusing on a senate run in the Philippines and Floyd focusing on promoting his fighters.

But Arum says, "If either man agrees to fight, my feeling is that they won't be able to resist."