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.