Tua Tagovailoa is expected to be interviewed early this week in the NFL and NFL Players Association's joint investigation of the injured Miami Dolphins quarterback's quick return to last weekend's game against the Buffalo Bills, sources told ESPN's Adam Schefter.

The league and players' association indicated Saturday that their review is ongoing while adding they agree that "modifications to the concussion protocol are needed to enhance player safety."

A league official told Schefter that the investigation is expected to last another week or two, and the results will be announced "almost immediately after."

Tagovailoa is expected to return to the Dolphins' facility Monday, sources told Schefter, as he continues to go through the concussion protocol after the head injury that forced him to leave Thursday night's game against the Cincinnati Bengals. Images of Tagovailoa's brain are being sent to three independent doctors to obtain as many opinions as possible about his condition.

Tagovailoa initially seemed to exhibit concussion symptoms after the hit to his head late in the first half of Miami's Week 3 home game against Buffalo, but he was cleared by a team physician and an unaffiliated neurotrauma consultant to return in the second half. Tagovailoa and the team later said his legs were wobbly because of a back injury.

After the Thursday hit, in which 6-foot-3, 340-pound Bengals defensive tackle Josh Tupou slammed Tagovailoa backward into the turf, the quarterback's hands froze up and his fingers flexed awkwardly in front of his face mask for several seconds as he lay on the ground 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 Dolphins hours later.

It's unknown whether there was any correlation between the two injuries. Dolphins coach Mike McDaniel said Friday that Tagovailoa will be out indefinitely.

The NFLPA fired the unaffiliated neurotrauma consultant on Saturday, saying in a joint statement with the league that they "anticipate changes to the protocol being made in the coming days based on what has been learned thus far in the review process." Sources told ESPN that the firing came after it was found the consultant made "several mistakes" in his evaluation of Tagovailoa.

There are three unaffiliated neurotrauma consultants at each game; they are jointly paid by the league and the players' union to work with team physicians to diagnose concussions. The NFLPA and the league each have the right to fire a UNC without agreement from the other party.

McDaniel has defended the Dolphins' handling of Tagovailoa's injury against the Bills, when he took a hit from linebacker Matt Milano and appeared to knock his head on the turf. McDaniel reiterated Friday that Tagovailoa was cleared by several layers of medical professionals during that game and said the third-year quarterback did not have a head injury, which is why he was not in the concussion protocol heading into Thursday's game.

ESPN's Marcel Louis-Jacques and The Associated Press contributed to this report.