Concussions have become one of the most worrisome aspects for professional football, but what happened to Russell Allen, takes the brain injuries to another level. Allen, a linebacker for the Jacksonville Jaguars, got "his bell rung" in a Week 15 game last season, but believing it to be a relatively minor incident, he stayed in the game. It turns out that it wasn't just another knock on the head — Allen had a stroke. And now his career is over as a result.

In a profile from Sports Illustrated's MMQB site, Allen reveals the details of his injury — the events leading up to it, the immediate aftermath, and what it means for his future. The short version of it: A head-on collision with another player left a small part of Allen's brain "dead," and he will never play in the NFL again.

Here's how Allen describes the hit that caused a dime-sized "dead spot" to form on his cerebellum, the part of the brain that impacts basic motor function and coordination:

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.

No big deal. Allen would play the rest of the game, making tackles and risking (unbeknownst to him) further seriously damaging his brain. When he started to have double-vision in the second half, Allen asked a teammate on the sideline if his eye looked OK. The teammate, linebacker Paul Posluszny, said yes, and Allen went back on the field. That game on December 13, 2013 against the Buffalo Bills was Allen's last.

Later that night, Allen says he suffered a headache and sensitivity to light — so much so that he couldn't watch television. When the headache persisted into the next morning, he told his team doctors, who scheduled him for tests. An MRI revealed Allen had a stroke during the game. He was lucky it wasn't worse, considering he kept playing after the initial injury. 

Does Allen regret finishing the game? He tells SI

If I could go back in time I would do it differently,” Allen says. “Being in it and knowing how I felt in that moment, the game feels so serious. You’re thinking, I can’t come out, because what if someone else comes in and takes my job? Or they need me out there, and I can’t come out because I really want to win. But you can’t do that, and I learned that the hard way.

But he doesn't blame the NFL or sport itself. "I have no motive to say it needs to change. For generations football has been tough guys, and that’s how it has to be. But there’s a fine line," Allen says. He believes in education, so that players are able to identify serious injuries during a game, and take the necessary precautions. 

Still, his NFL career is over. Allen spent the rest of the 2013 season on injured reserve for what was labelled a "concussion" and was cut by the Jaguars earlier this year for a "failed physical." 

As the NFL is underpaying for concussion settlements, Allen's story is a reminder of the long-term damage that players risk; the permanence of Allen's stroke is not in question. He doesn't blame football for what happened to him, but it's telling that why Allen felt obligated to keep playing. "Tough guys" finish the game, no matter what, because if you aren't willing to take the hit, someone else is. That's the culture the NFL breeds. Allen's story won't put an end to the NFL any time soon, though, but it will certainly raise more questions about what the league is doing for player safety. The league has intensified the protocol for determining when players have suffered concussions and cannot return to action, but it appears that Allen never consulted with team doctors during the game.

One last fascinating point from Allen's profile: his injury came with a bit of foreshadowing. He won the starting linebacker position after taking over in a game for Clint Session. Why did Session exit that game? He suffered not one concussion, but two.