The NFL will re-examine its concussion protocols after concerns over the return to play of Miami quarterback Tua Tagovailoa after a head knock, before his removal on a stretcher from his next game four days later.

In a joint statement, the NFL and the NFL Players Association (NFLPA) said an investigation into the "application of the concussion protocol" involving Tagavailoa remained ongoing, and that no conclusions had been reached about protocol violations or medical errors.

"The NFL and the NFLPA agree that modifications to the concussion protocol are needed to enhance player safety," the organisations said in the statement.

"The NFLPA's Mackey-White Health and Safety Committee and the NFL's Head Neck and Spine Committee have already begun conversations around the use of the term 'gross motor instability' and we anticipate changes to the protocol being made in the coming days based on what has been learned so far in the process.

There are three unaffiliated neurotrauma consultants (UNC) at each game. They are jointly paid by the league and the players' union to work with team physicians to diagnose concussions.

Tagovailoa initially seemed to exhibit concussion symptoms after the hit to his head during the home game against Buffalo, but he was cleared by a team physician and UNC to return.

He and the team later explained his legs were wobbly because of a back injury.

After the hit in the following game, when 190cm-tall, 154kg Bengals defensive tackle Josh Tupou slammed him backward into the turf, Tagovailoa's hands froze up and his fingers flexed awkwardly in front of his face mask for several seconds as he laid on the turf in Cincinnati.

He remained on the ground for several minutes until he was taken away on a stretcher and sent to a hospital. He was released from the hospital and flew home with the team hours later.

It's unknown whether there's any correlation between the two injuries. Concussions are common in the NFL, especially when a player is thrown to the ground and his head hits the turf.