51-year-old Lynn spelled it out on the season premiere of HBO's "Hard Knocks" -- and though he didn't specifically say WHEN he tested positive.

"I can't promise you you're not going to get infected," Lynn told his players during a video chat meeting ... "I got infected."

He continued, "What I want to do is limit your exposure, but then when that whistle blows, let's go kick somebody's ass and play some football."

Later in the show, Lynn was asked about his symptoms -- and how he realized he had coronavirus.

"I’m coughing a little bit, then my body started aching a little bit. I couldn’t get comfortable in my bed," Lynn said.

Lynn then explained that he was watching a golf match and saw that a golfer had pulled out after experiencing similar symptoms -- and laster tested positive for COVID.

"I said, 'sh*t. My body kinda felt like that.'"

"If I hadn't been watching the golfing game and saw that golfer complain about backaches and soreness ... I never even would have gotten tested And I would have had this and never even known it. And probably got people infected."

Lynn says he was more worried about being branded an "outcast" than batting the virus itself.

"All the sudden, I'm the problem. I'm used to fixing the problem, now I'm the problem."

Lynn was eventually cleared to return to work -- where his players are staff are tested for COVID every day.