Miami Dolphins quarterback Tua Tagovailoa underwent additional testing Friday and remained in the concussion protocol a day after hitting his head against the turf during a game, a frightening moment that has sparked controversy about how the league handles potential head injuries.

Tagovailoa had a headache Thursday night and Friday morning, Dolphins coach Mike McDaniel said. The team was waiting still for results from the MRI, which McDaniel said was done out of “extra precaution” on top of the CT scans and X-rays taken the night before.

McDaniel said he had no timetable on when Tagovailoa might be able to return.

“I’m not even really thinking about timetables or anything regarding him as a player right now,” McDaniel said. “It’s all about Tua the person.”

Tagovailoa tweeted a statement Friday evening, thanking fans for their support. 

“It was difficult to not be able to finish the game and be there with my teammates, but I am grateful for the support and care I’ve received from the Dolphins, my friends and family, and all the people who have reached out,” he said. “I’m feeling much better and focused on recovering so I can get back out on the field with my teammates.”

Tagovailoa was sacked by Bengals defensive tackle Josh Topou late in the first half Thursday. On the play, he spun awkwardly and was thrown to the turf. While on the ground, Tagovailoa appeared to display the fencing response, with his fingers frozen in front of his face.

He remained down for more than seven minutes before being loaded onto a backboard and stretchered off the field. He was taken to University of Cincinnati Medical Center for evaluation.

The Dolphins later said he was conscious and had movement in all of his extremities. He was discharged Thursday night and flew back to Miami with the team.

McDaniel said Tagovailoa was interacting with teammates on the flight home. He sat next to McDaniel and talked to him about the game.

“His personality was normal Tua,” McDaniel added.

It was unclear what is next for Tagovailoa, whose breakout season has been interrupted by a scary injury two weeks after he threw a career-high six touchdowns in Week 2 against Baltimore.

Last Sunday against Buffalo, Tagovailoa took a hit from Bills linebacker Matt Milano late in the first half and appeared to hit his head on the turf. He stumbled when he got up and was immediately taken to the locker room, where he was evaluated for a concussion. He returned to the game at the start of the third quarter and was not in the concussion protocol the following week, despite questions about why he was allowed to return to the game at all.

The Sunday incident prompted the NFL and National Football League Players Association to jointly review the extent of his injuries and if the Dolphins followed proper concussion protocol that day.  

“Ninety-nine point nine percent of the time that player is ruled out of the game without even thinking,” CEO of the Concussion Legacy Foundation Chris Nowinski told “CBS Mornings” co-host Nate Burleson. 

Nowinski, a neuroscientist, said that the Dolphins put Tagovailoa’s life in danger and failed to protect him. 

“This is a failure by everybody. This is a failure by the medical team. This is a failure by the independent medical team on Sunday. This is a failure by the coaching staff. The problem is all these failures, none of them are going to be punished. The person who is punished here is Tua,” Nowinski said. 

The results of the joint investigation between the NFL and the NFLPA have not yet been released, but Miami Dolphins head coach Mike McDaniel is adamant that the Dolphins followed proper protocols last Sunday. 

“If there’s any sort of inclination that someone has a concussion, they go into concussion protocol, and it’s very strict. As long as I’m the head coach that will never be an issue that you guys have to worry about,” McDaniel said.  

The NFLPA tweeted Thursday night: “Player health and safety is at the core of the union’s mission. Our concern tonight is for Tua and we hope for a full and speedy recovery. Our investigation into the potential protocol violation is ongoing.”  

McDaniel reiterated Friday that Tagovailoa was cleared by several layers of medical professionals during that game and said he did not have a head injury.

“My job as a coach is here for the players. I take that very serious,” McDaniel said. “And no one else in the building strays from that.”

He added: “There was no medical indication from all resources that there was anything regarding the head. If there would have been, of course, I wouldn’t have been able to live with myself if I prematurely put someone out there.”

Reaction Thursday came swiftly from around the NFL. Chiefs quarterback Patrick Mahomes and Broncos QB Russell Wilson promptly tweeted with concern for Tagovailoa’s well-being.

“Praying for you Tua,” Wilson wrote.

Some criticized the decision to play Tagovailoa so soon after his injuries in Sunday’s game.  

Baltimore Ravens head coach John Harbaugh called the situation “astonishing.”

“I couldn’t believe what I saw last night,” Harbaugh told reporters Friday. “I couldn’t believe what I saw last Sunday. It was just something that was astonishing to see, and I’ve been coaching for 40 years…I’ve never seen anything like it before.”

Hall of Famer Shannon Sharpe tweeted: “That’s a serious injury . Tua shouldn’t have been out there with Sunday Thursday turn around. Sometimes players need protecting from themselves. Dolphins failed Tua.”