INDIANAPOLIS — A few minutes into the fourth quarter Saturday night, the fans at Lucas Oil Stadium stopped watching the Bears’ preseason game against the Colts.

“They were looking at their phones, and they were just wide-eyed,” Bears cornerback Prince Amukamara said. “Everyone’s in disbelief because it’s so sudden.

“Yeah, it’s never happened. Just never seen anything like that.”

Colts quarterback Andrew Luck — Peyton Manning’s successor who became a four-time Pro Bowl player in seven injury-filled seasons and who was supposed to star for the team again this year — was retiring, ESPN reported.

Luck, who was on the sideline in shorts and a T-shirt, knew this day was coming, but he kept it a secret. He was supposed to announce his retirement Sunday, but the report hastened things.

He told his teammates in the locker room Saturday night. About a half-hour after the Bears beat the Colts 27-17, Luck was in front of a podium. Still wearing a T-shirt and shorts and with notes in hand, he made official the most stunning NFL retirement in years.

Luck, who turns 30 next month, broke down in tears three times.

“Honestly, it’s the hardest decision of my life,” he said. “But it is the right decision for me. For the last four years or so, I’ve been in this cycle of injury, pain, rehab.

“It’s been unceasing and unrelenting, both in season and offseason. I felt stuck in it. The only way I see out is to no longer play football. It’s taken my joy of this game away.”

Luck had missed 26 games since hurting his shoulder in 2015. A calf strain cost him the Colts’ minicamp, and he didn’t play a single training-camp practice in full pads. The calf problem, combined with myriad ankle issues, made playing impossible, he said.

He explained his retirement in nine words — “Lack of progress on my ankle . . . I’m in pain” — but it runs deeper than that.

“After 2016, when I played in pain and was unable to regularly practice, I made a vow to myself that I would not go down that path again,” Luck said. “I find myself in a similar situation, and the only way forward for me is to remove myself from football and this cycle that I’ve been in.”

Luck knew for the last 10-14 days that retirement was likely. He told himself to enjoy a strenuous pregame workout a week earlier — it was the last time he would do so at Lucas Oil Stadium. When he hugged Bears defensive coordinator Chuck Pagano — his head coach from 2012 to ’17 — before the game, he knew he would never be on the field as a player again.

“It’s maybe fitting that Chuck was the coordinator for the opposing team tonight,” said Luck, the No. 1 overall pick in 2012.

While Luck knew his secret, the fans did not. When he and his teammates jogged off the field, fans booed him.

“It hurt,” Luck said.

Luck’s bosses — coach Frank Reich, general manager Chris Ballard and owner Jim Irsay — had red eyes and thousand-yard stares while he spoke. They knew the day was coming, but that didn’t make it easier.

Irsay oddly sounded as if he still didn’t

believe it.

“He’s leaving on the field $450-$500 million on the table, potentially,” he said. “I mean, half a billion dollars! And he’s saying, ‘You know what, I want to have . . . my integrity has to be real.’ ”

Irsay called the retirement a transformative situation — “A worm becoming a beautiful butterfly,” he said, bungling how nature works — before trying to will the retirement not to be. He said he wouldn’t be surprised if Luck returned to football when he felt better.

“I don’t rule it out,” he said, “because as quickly as this thing descended on us and as mysterious as it was coming upon us, it could leave the same way.”

Luck, though, was committed.

“I’ve come to the proverbial fork in the road,” he said. “I made a vow that if I ever did again, that I would choose me, in a sense.”

Contributing: Jason Lieser