This is no ordinary stroke either: he has a dead spot on his cerebellum -- aka the lizard brain -- the size of a dime. He finished the game after the stroke. So much for NFL brain injury detection during games.

He finished the game?! Wow that is hardcore!

It went dark on Dec. 15, 2013. Just after halftime in a Week 15 game against the Bills, Allen strafed across the hash marks in pursuit of a ballcarrier before running into center Evan Wood several yards downfield. The two collided face-to-face, dead center from Allen’s perspective, and Allen walked away buzzed as he casually shrugged his shoulders.

“It was strange because it was so routine,” Allen says. “We hit, I got off the block, no big deal. I felt something flash—like they say when you get your bell rung. I didn’t lose consciousness. I walked back to the huddle and finished the drive.”

He showed no immediate signs of being injured. One snap later he sprinted toward the right sideline as running back C.J. Spiller pulled down a swing pass. A half-step too late, Allen exalted cornerback Alan Ball for making the tackle, slapping him on the helmet and shouting, “I see you A.B.!” Two snaps later Allen dropped back in coverage on 3rd-and-seven, only to switch gears as quarterback E.J. Manuel escaped from a crumbling pocket. Allen made a solo tackle in open field, wrapping up Manuel’s ankles after a two-yard gain and forcing a punt on fourth down.

At some point in the second half, however, Allen began having double vision on the sideline. He went to fellow linebacker Paul Posluszny for an evaluation. “I remember him telling me to look in his eyes,” Posluszny says. “He kept asking, ‘Is my eye OK?’ I looked at him and said, ‘Yeah, it looks fine.’ I couldn’t tell the seriousness of the situation. How do you tell?”

For Allen, this is the embarrassing part. He went back in and finished the game.

Afterward, he showered with a headache, got dressed with the same headache, drove home with his wife, struggled to watch Sunday Night Football because of light sensitivity, and went to bed with the same headache. If he still had it when he woke up, he told himself, he’d inform the team. The headache persisted on Monday morning, so he told the trainers that he’d been dinged on Sunday and that for a few minutes he’d seen double on the sideline.

Big red flag.