WHITE SULPHUR SPRINGS, W. Va. >> The reality of life playing football is that major knee injuries are common.

What isn’t: Tearing your ACL and then playing three-plus quarters of football after.

That’s what happened to Broncos inside linebacker and captain Alex Singleton on Sunday against Tampa Bay, dealing a significant blow to a stout Denver defense just three weeks into the season.

Head coach Sean Payton and Singleton’s teammates reacted with equal parts disappointment and amazement at the season-ending injury, its impact and Singleton’s ability to play 49 snaps after sustaining what appeared to be a non-contact injury during safety Brandon Jones’ first-quarter interception return.

“If there’s a story or something that could — if I had to explain Alex in one thing, I would just point that out,” rookie quarterback Bo Nix told reporters at The Greenbrier here after Denver’s Wednesday practice. “Alex is the type of guy who would tear his ACL on the (eighth) play of the game and go out there and finish the game.”

Singleton dropped back into coverage on a third down and hopped in apparent pain in the open field. He ran down to try to block somebody on the return and limped off the field after the play. He told some teammates, including Nix, about his knee being sore at halftime, but he played all 57 defensive snaps.

After the game, he reported knee soreness to the Broncos training staff and they did the ACL structure test. Vice president of player health and performance Beau Lowrey told head coach Sean Payton he felt some movement — a bad sign for the structure of the knee — but couldn’t tell for sure. Imaging later that evening confirmed the tear.

“(Singleton) was just as surprised,” Payton said.

Singleton is with the team here and will have surgery likely sometime in early October after the swelling recedes.

The injury ends a budding Iron Man streak for Singleton, too. He had not missed a play for the Broncos since Week 3 in 2023. From the start of Week 4 against Chicago last year, he played 1,115 straight snaps over 17 straight games.

Not only that, but he told The Post recently that over his nine years playing pro football — three in the Canadian Football League and now six in the NFL — he’d never missed a single day of practice.

Now he’s going to miss several months’ worth.

He has totaled 371 tackles in 37 games (31 starts) since signing with the Broncos before the 2022 season.

Singleton signed a three-year deal worth up to $18 million ahead of the 2023 season and through three games this fall has led the team with 31 tackles. He also logged an interception in the season-opener against the Seattle Seahawks.

“We’re going to miss him out on the field,” Surtain said. “… I’ve got to step up my leadership role even more losing a presence like that. Just rally behind the whole defense and making sure we’re on the same page and making sure the standard stays the same. Making sure we execute the same way. Losing Alex is going to be tough, but we’ve got guys stepping in that are going to be key components to our defense.”

The Broncos signed a pair of veteran inside linebackers to the practice squad on Wednesday in Kwon Alexander and Zach Cunningham. They’re expected to elevate undrafted rookie Levelle Bailey from the practice squad for game day against the New York Jets, a source told The Post on Wednesday.

Most of the burden of replacing Singleton, though, will fall to Cody Barton and Kristian Welch, who had formed a three-man rotation with him over the first three games. Justin Strnad is also on the 53-man roster, though he’s been a special teams-only player since Payton arrived in Denver before the 2023 season.

“I’m not worried about who takes his spot,” Payton said. “It’s about everybody else picking up the slack.”