Russell Allen will never play football again.

The Jaguars linebacker confronted this reality last Thursday, sitting on the beige sectional in the living room of his home in San Diego’s northern suburbs. It was 9 a.m. in California and noon in Jacksonville, when his agent called with the dreadful news Allen had expected for months: The one-time rookie free agent had been cut after five NFL seasons. The Jaguars announced Allen’s release along with three other cuts, listing the official reason as a failed physical. But this was no ordinary roster move.

Coach Gus Bradley would call Allen to tell him how sorry he was—and that he would always be a Jaguar. General manager David Caldwell also would reach out to express his condolences. As the news sunk in, Allen’s wife, Ali, whispered instructions to their 2-year-old son. Parker did as he was told, running over to his father and telling him he did “a good job playing football.” Allen, 27, burst into tears.

In the back of the brain, the cerebellum tells the body how to walk, run and even crawl. It might also play a role in discerning happiness and fear, but the medical science isn’t exactly sure. What doctors are sure of, however, is that Russell has a spot on his cerebellum, no bigger than a dime, that is dead.

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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?”