Tua Tagovailoa was able to pass the NFL's sideline check for concussions on Sunday despite appearing wobbly on his feet — here's what the protocol looks like

• Tua Tagovailoa was diagnosed with a concussion after a scary hit against the Cincinnati Bengals on Thursday night.

• It was the second big hit Tagovailoa had taken in four days, having also shown signs of a potential concussion on Sunday against the Bills.

• The second hit brought re-focused attention to the first one, and whether or not the NFL's concussion protocols were properly followed on Sunday.

Miami Dolphins quarterback Tua Tagovailoa was stretchered and carted off the field on Thursday night after a scary hit left him stiff on the ground.

Tagovailoa's injury was immediately concerning to those watching, as it was the second big hit he had taken in just four days, having looked wobbly on his feet after being shoved to the ground by the Buffalo Bills on Sunday.

After the hit on Sunday, the Dolphins initially said Tagovailoa had sustained a head injury, and was questionable to return. He missed just three plays, and returned to the field after halftime to finish the game.

Head coach Mike McDaniel said afterwards that Tagovailoa had suffered a back injury, and that was the reason he had gotten up shakily.

"Tua, he went out with a lower back," McDaniel said. "He kind of got bent back pretty significantly on a quarterback sneak earlier. I was kind of with everyone else. When he hit his head on the ground, I assumed it was a head injury, but his legs got wobbly because his lower back was completely loose, and as he described it, he said his lower back was like Gumby or something."

Despite the Dolphins' insistence that it was a back injury, the NFLPA launched an investigation Monday into Tagovailoa's handling after the hit, and whether or not all the steps of the NFL's concussion protocol were properly followed before sending him back out onto the field.

The league's protocols for head injuries consist of two processes — one for diagnosing a concussion suffered during a game, and another to ensure a player who is diagnosed with a concussion doesn't return to the field until they have recovered as best they can.

According to the league's Concussion Game Day Checklist, the first process is initiated after a player receives a hit to the head and exhibits signs of a potential concussion. The process can also be initiated by several people watching the game, including referees, team physicians, as well as the unaffiliated neurotrauma consultants (UNCs), the independent medical professionals present to ensure impartiality in the decision-making process.

After a player has been identified as having potentially suffered a concussion, they then go through a "sideline survey," which includes a focused neurological exam to determine whether or not a concussion occurred.

At this point, several things can immediately deem a player a "no-go," including a loss of consciousness and amnesia. "Gross motor instability" can also knock a player out of a game for good, if it determined by team physicians, in consultation with the UNC, to be neurologically caused.

If a player passes the sideline survey, they can return to play. If results are inconclusive, the player is escorted to the locker room for further evaluation.

On Sunday, Tagovailoa was allowed to re-enter the game, indicating he got through the sideline check. After his hit on Thursday, Tagovailoa was obviously removed from the game. Given that he was officially deemed to have suffered a concussion against the Bengals, he now enters the second process of the league's protocols, which are meant to ensure a player doesn't take the field again until they are ready.

First, the player in question is prescribed rest, then slowly worked back into aerobic activities under the direct oversight of the team's medical staff. From there, players can begin football-related exercise, and then brought back into non-contact team drills, before finally getting cleared for contact once again.

There is no exact timetable for clearance, and as things stand, McDaniel isn't offering one.

"I'm not even thinking about timetables as a player," McDaniel said on Friday. "It's about Tua as a person. We're just worried about him getting healthy and getting all of the testing done. We'll cross the bridge on timetables."

Given the severity of Tagovailoa's second hit, it's possible that his season is over.

"If I'm Tua, I'm not going back this season, because God forbid you have three concussions in 12 weeks," Chris Nowinski, co-founder and CEO of Concussion Legacy Foundation, told Insider on Friday. "That just doesn't happen anymore in modern medical society."

While the Dolphins insisted that it was a back issue that caused Tagovailoa's initial stumble on Sunday, Nowinski was certain that it was a concussion, making the second hit Tua suffered all the more troubling.

"Now we have to wait and watch if the Dolphins will finally admit that it was a concussion last week," Nowinski said. "Because if they tried to lie again and put him back in, we're basically going to watch and just hope he doesn't die. You're going to be pretty sure you're going to be taking years off the end of his career, or at least impairing his performance going forward.

"I hope Tua has a full recovery," Nowinski said. "But if I was Tua, I would not trust this team anymore."

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