Tua Tagovailoa left the Dolphins Week 4 game on a stretcher with a head and neck injury.
After Tagovailoa hit the ground, his hands stiffened in an unnatural position.
Neurologist and concussion expert Chris Nowinski said Tagovailoa showed signs of "decorticate posturing," a severe, "primitive" brain response that indicates damage to the cortex.
During the second quarter of the Thursday Night Football game between the Miami Dolphins and Cincinnati Bengals, Dolphins quarterback Tua Tagovailoa was pulled to the ground and slammed his head against the turf.
Almost immediately, Tagovailoa's hands stiffened, with his fingers crossed, in a disturbing image.
Tagovailoa was eventually carted off the field and taken to the hospital with head and neck injuries. The Dolphins later reported that Tagovailoa had movement in all of his extremities. He was discharged and flew home with the team.
In the hours since the devastating hit, there has been debate over whether Tagovailoa's seized-up hands showed that he experienced a "fencing response" or "decorticate posturing," two brain responses to trauma.
According to Chris Nowinski, a neuroscientist and CEO of the Concussion Legacy Foundation, Tagovailoa had decorticate posturing.
"You usually see it in stroke when you've had massive damage to the cortex," Nowinski told Insider. "It's basically like a primitive movement controlled by the brain stem."
He added: "Basically it's made his cortex no longer in control of his body, and his brain stem took over. And that's what it does when you have a damaged cortex."
Making matters worse, Tagovailoa had appeared to injure his head after a hit on Sunday against the Buffalo Bills. Tagovailoa wobbled as he stood back up and was unable to stay on his feet without assistance. Yet the 24-year-old quarterback returned to the game after halftime.
Though the Dolphins said Tagovailoa had cleared concussion protocol and actually had a back injury, the NFL Players Association launched an investigation into what had occurred.
Nowinski said that he had initially believed Tagovailoa showed fencing posture, but after consulting with other experts, determined it was decorticate.
Still, there isn't consensus. Dr. Allen Sills, the NFL's Chief Medical Officer, told NFL Network that Tagovailoa had "fencing posture."
Nowinski said decroticate posturing can affect a person's being.
"It's serious," Nowinski said. "It's deep involvement in the brain. It can really change, you know, who you are, how you feel."
On Friday, Dolphins head coach Mike McDaniel defended the team's handling of Tagovailoa - from keeping him in the Week 3 game against the Bills and in the run-up to Thursday's game.
"I had no worries whatsoever," McDaniel told reporters. "I'm in steady communication with this guy. There were absolutely no signs. He had no head injury symptoms whatsoever. There was no medical indication from all resources that there was anything regarding the head. If there would have been anything lingering with his head, of course I wouldn't have been able to live with myself if I prematurely put someone out there in harm's way."
McDaniel said Tagovailoa was awake and responsive during the team's flight, saying he watched the movie "MacGruber."
Even if he is improving, Nowinski said Tagovailoa should not play again this season.
"We need to err on the side of caution. At least I hope we start erring on the side of caution now," Nowinski said. "If he's back this season again, they're rolling dice with this future for no reason at all."