We saw Manny Pacquiao age in the ring Saturday night. We saw the legs lose their mobility as a younger, more aggressive opponent roughed him up with head butts and headlocks. And when the decision was announced, the world was shocked as the underdog had his hand raised in victory. It couldn’t have worked out better for Floyd Mayweather and Conor McGregor.

Pacquiao’s stunning defeat to a seemingly overmatched Jeff Horn in Brisbane, Australia, feeds into those who believe McGregor might do to Mayweather what Horn did to Pacquiao. The Aussie came out swinging, and though he landed just 15 percent of his 600-plus punches, he won enough rounds to earn a unanimous, if unpopular, decision.

“That couldn’t have worked out better for Floyd and Conor,” said a rival promoter, not connected to the Mayweather-McGregor fight. “Now they can say, ‘Look what happened to Pacquiao. He got old in the ring. The same could happen to Floyd.’ ”

Mayweather is 40. McGregor, the UFC’s biggest star, is 28. Though there virtually is no way the mixed martial arts champion can match the boxing skills of Mayweather, McGregor’s best chance is to force a fight the way Horn did. Or at least that will be one of the strategies talked about this month when the two fighters finally come together for a promotional tour.

There was an unconfirmed report on social media the first stop would be Wembley Stadium in two weeks to rev up the UK audience. Other stops are expected to be made in Ireland, New York, Los Angeles and Las Vegas. The fight could turn out to be one-sided and boring, but the press conferences figure to be YouTube classics.

The Pacquiao-Horn fight was seen by 4.4 million viewers on ESPN, making it cable television’s highest-rated and most-watched boxing telecast since 2006 and ESPN’s highest-rated boxing broadcast since 1995.

A crowd of 51,052, the largest ever to watch a boxing event in Australia, was in attendance. The 800-pound elephant in the room was Mayweather-McGregor. Pacquiao had hoped an impressive showing might earn him a rematch with Mayweather in the near future. After Horn won, he called out the former pound-for-pound king who retired two years ago.

Mayweather-McGregor easily will surpass Mayweather-Pacquiao as the richest fight in boxing history. Mayweather-Pacquiao generated $600 million for their 2015 match won by Mayweather. Insiders have told The Post that Mayweather-McGregor could approach $1 billion world-wide.

Pacquiao, meanwhile, is left to contemplate retirement. A sitting senator in the Philippines, Pacquiao already has a busy political career. His trainer, Freddie Roach, seems in favor of his fighter retiring, though Pacquiao seems to want a rematch with Horn.

“I said, ‘Well, maybe a rematch, but then that’s it,’ ” Roach told Yahoo Sports. “There can be no more [fights with] Mayweather and so forth.”

Whether McGregor, who never has boxed professionally, has a chance against Mayweather depends on whom you talk to. Followers of MMA believe the intangibles favor the Irishman — who will be bigger, faster, younger and fighting with nothing to lose. Boxing purists see Mayweather dominating an amateur.

What is certain is McGregor backers can now point to Pacquiao’s loss to Horn to make their case for an upset.