Miami Dolphins quarterback Tua Tagovailoa lies on the ground after colliding with a Bills defender during a game on Thursday night.

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It was a routine run for a first down, the kind quarterbacks make every week in the National Football League.

But Miami Dolphins forward Tua Tagovailoa leaned head first into the tackle, and the impact on the defender’s chest sent his head hard to the left, sending him crashing to the ground with his right fist clenched and raised in the air, which appeared to be a telltale sign of a brain injury.

Later in Thursday night’s game, the Miami Dolphins made it official: Tagovailoa had suffered a concussion, the latest for the 26-year-old whose career had already been defined by his troubling history of brain injury.

Tagovailoa’s concussion has prompted dramatic reactions from fans and former players who are urging him to retire, reflecting the growing awareness among the American public of the risks of long-term brain damage that can result from blows to the head.

“If I were him, I’d be seriously thinking about quitting football right now. If that were my son, I’d say, ‘Maybe it’s time.’ These are the kinds of things you don’t play with,” said Tony Gonzalez, a Hall of Fame tight end for the Kansas City Chiefs and Atlanta Falcons, speaking after the game as an analyst on the Amazon Prime broadcast.

On social media, former players and fans shared the same sentiment. “He needs to retire because of his health issues over his longevity,” wrote Dez Bryantthe retired Dallas Cowboys wide receiver.

“I really hope Tua is OK, but he’s got to think seriously about sitting it out,” said Shannon Sharpe, the former Denver Broncos tight end. “His concussions are getting worse and he’s a young man with his whole life ahead of him.”

Concussions and repeated blows to the head are important risk factors for long-term brain injury, including chronic traumatic encephalopathy (CTE), a serious condition that can cause premature dementia and death.

“There is no magic number of concussions that will necessarily end your career,” said Chris Nowinski, a former football player and professional wrestler who went on to study neuroscience and now heads the Concussion Legacy Foundation.

“It all depends on how he recovers from this, but too many concussions can lead to chronic symptoms and psychological disorders,” Nowinski continuedwriting on the social media site X. “He is in a very difficult position.”

A small number of NFL players have left the sport early due to concerns about their neurological health, including Carolina Panthers All-Pro linebacker Luke Kuechly and San Francisco 49ers linebacker Chris Borland, who retired after just one season in the NFL.

“With Tua, there’s the added pressure of being the star of the franchise, the player they build around,” Borland told NPR in 2022. “It’s vital that players have better protections because if they were left to their own devices, to a hundred different incentives, we would do anything to get back on the field.”

Tagovailoa left the field on his own after suffering a concussion during Thursday night’s game against the Buffalo Bills.

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In 2022, the NFL revised its protocol for evaluating concussions after Tagovailoa was cleared to play again after suffering a head injury while being medically evaluated. In another game just four days later, he hit his head again, this time receiving a formal diagnosis of a concussion. Weeks later, he suffered another concussion and missed the remainder of the season.

Before the 2023 season, Tagovailoa had devoted his offseason to becoming less susceptible to concussions, taking jiu jitsu classes and training on how to roll his body in the event of a fall, rather than letting his head hit the grass.

The season that followed was the best of his career. He led the league in passing yards and led the Dolphins to an 11-6 record, their most wins since 2008. In July, he was rewarded with a $212 million contract extension.

If he decides to retire, the amount of compensation he receives will depend on whether he is medically cleared to play football again.

If he is cleared to play but still retires, Tagovailoa will lose out on $124 million. If he is medically forced to retire, he will collect that money, according to Spotraca service that keeps track of professional sports contracts.

After the impact during Thursday’s game, Tagovailoa walked off the field unaided. The Dolphins said he was able to speak with medical staff in the locker room.

On Friday, head coach Mike McDaniel declined to speculate on a timeline for Tagovailoa’s return. “The best thing I can do is not try to assess what this means from a football standpoint,” McDaniel said. “Trying to meet (timelines), trying to assess what this means from a game perspective — this is heavy stuff.”

As of Friday morning, Tagovailoa had not publicly commented on his options.