NEW YORK — From the second he sent an Instagram message to Anthony Joshua’s promoter pushing to be the replacement opponent, Andy Ruiz Jr. has been making this up as he goes.

So, when his shocking seventh-round technical knockout of England’s three-belt champion Saturday at Madison Square Garden suddenly made him an inspiration as the first heavyweight champion of Mexican descent, Ruiz embraced the role.

“I had a lot of fans over there rooting for me,” Ruiz said of his border farming hometown of Imperial, Calif., east of San Diego. “I’m sure they’re all proud. That’s where I grew up. I’m sure they’re all smiling, saying, ‘He did it, a kid from a small, little town did it.’

“Everything is possible, man. You believe in God, focus, believe in your dreams, never give up. And if someone puts you down, don’t listen. Keep working. Look what I did. I just made history.”

Ruiz (33-1, 22 knockouts) turned in a captivating performance that switched the coronation of Joshua’s U.S. debut into a celebration of the human spirit. The pudgy fighter who’d been beaten in his previous title fight knocked down the far fitter Joshua (22-1) four times en route to winning the World Boxing Association, World Boxing Organization and International Boxing Federation belts.

“It’s an upset. One shot on top of the dome rattled me quite a bit,” Joshua surmised after undergoing a post-fight concussion screen. “The better man won.”

Joshua’s loss shakes up the heavyweight division and adds another interesting date to the calendar, as Joshua’s promoter Eddie Hearn said he expects his fighter to invoke the rematch clause in his contract to meet Ruiz again around November, likely in Cardiff, Wales.

That date will be sandwiched between World Boxing Council champion Deontay Wilder’s fall rematch with Luis Ortiz, and Wilder’s planned early 2020 rematch with unbeaten former three-division champion Tyson Fury.

The interesting wrinkle for Joshua, who has one fight left on his contract with the streaming service DAZN, is that he has been reduced from a heavyweight who called all the shots after drawing 90,000 to Wembley Stadium two years ago to a fighter who must take the rematch.

Otherwise, Ruiz, who like Wilder fights for Al Haymon’s Premier Boxing Champions, has contractual leeway to take his belts and move toward Wilder down the road.

“It’s a minor setback,” said Joshua, who’s 29 like Ruiz. “I’ll bounce back and get my hands on those belts again. … I’ll beat him up. Tidy up, brush off the cobwebs and fight again.

“Losing is something we can learn from. Tighten up. Get smart. There’s more to come. Winning is everything, but if you do happen to take an ‘L,’ reset, readjust … I do not condone losing. On to the next one. The hunger is always there. I’m ready to go back to work tomorrow.”

Joshua has work to do now as critics question his ability to take a punch. Joshua was wobbled and never the same after getting decked twice in the third round. It appeared referee Mike Griffin did Joshua a favor by extending the eight-count on the second knockdown that round.

Despite his muscular, fit frame, Joshua was fatigued whenever Ruiz began to land blows on him after that, and a 12-punch flurry resulted in another seventh-round knockdown followed by the ending, when Joshua arose, but leaned back on his corner pole and couldn’t satisfy Griffin that he was ready to resume.

Griffin asked, “How you feeling, champ?” Joshua responded twice, “I’m good.” Griffin waved his arms it was over and Ruiz jumped in joy.

“I should’ve never touched the canvas,” Joshua lamented.

For Ruiz, it was an opportunity to highlight that appearances can be deceiving.

“They looked past me. They should’ve looked at me in the first place instead of (initial opponent Jarrell Miller) and Deontay Wilder or Tyson Fury,” Ruiz said. “The way I look, the extra flab that I carry” is why people underestimate him. “Now, I want to get in really good shape and look like a Mexican Anthony next time we fight.

“We made it happen. No one can take that from me. Nobody. I just shocked the world. Now, I’ve got to work even harder and make a legacy. I’m not going to let these belts go.”

Hearn said Ruiz, remarkably, can position himself as “the golden goose” of the sport by repeating a victory over Joshua, and he expects that second act to occur as scheduled.

“That was a night at the Garden you will never forget. A.J. was absolutely devastated, but he will come back stronger,” Hearn said.

“Andy Ruiz won fair and square. I know Anthony Joshua’s work ethic. I know this will burn him so bad inside because he’s the ultimate competitor. Great fighters come back and improve. The future will show what Anthony Joshua is made of.”

That may be true, but Saturday night showed what Andy Ruiz Jr. is made of, and as he displayed by rising from an early third-round knockdown and reacting forcefully to each of Joshua’s best punches, there’s more to come.

“Everyone’s been doubting me from the beginning,” Ruiz said. “I’m here now, the first Mexican heavyweight champion of the world.”