When the Jaguars released Russell Allen last week, it came as a major surprise: Allen was a starting linebacker and only 27 years old, and looked like an important piece of the team the Jaguars are trying to build.

Now we know why Allen is done with the Jaguars, and why he’ll never play football again.

Allen told Robert Klemko of TheMMQB.com that he suffered a stroke on the field during Jacksonville’s Week 15 game against Buffalo. At the time, Allen sensed something was wrong, but he didn’t think it was a big deal and actually stayed in the game. But neurological examinations have revealed that Allen has a dead spot in his brain and can’t risk playing a violent sport like football.

“It was strange because it was so routine,” Allen said. “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.”

Allen sounds at peace with his decision to leave the game, but it’s a frightening story nonetheless. NFL players are so tough that they’ll keep playing through anything. Even a stroke.