Following a hit that left Miami Dolphins quarterback Tua Tagovailoa stumbling to the ground and requiring help to stand in a win over the Buffalo Bills, the NFL Players Association has initiated an investigation into how Tagovailoa was cleared to continue playing Sunday.

A league source familiar with the union’s request, which is authorized by the collective bargaining agreement, said the NFLPA is seeking information on the steps taken by the Dolphins after Tagovailoa’s head bounced off the ground following a hit by Bills linebacker Matt Milano late in the second quarter. Tagovailoa rose to his feet under his own power, but appeared to shake his head lightly before stumbling and falling back to the turf. He was eventually helped to the locker room by athletic trainers, while backup quarterback Teddy Bridgewater stepped in for the rest of the half. The Dolphins announced in the press box and on Twitter that Tagovailoa was questionable to return to the game with a “head injury.”

That announcement, followed by Tagovailoa’s surprising return after halftime, lit up Twitter and caught the attention of union leadership, who contacted the NFL’s league office to initiate an investigation into the sequence of events and the injury protocols that the Dolphins addressed. Following Tagovailoa’s return, Miami went on to beat the Bills 21-19.

Following the game, the Dolphins said Tagovailoa told them he had suffered a back injury on the play prior to Milano’s hit.

“When he hit his head on the ground, I assumed it was a head injury,” Dolphins head coach Mike McDaniel said. “But his legs got wobbly because his lower back was completely loose as he described it.”

Tagovailoa reiterated that explanation to reporters, adding that he had passed “whatever concussion protocol” that had been administered.

“Kind of got my legs caught under someone [the previous play], they were trying to push back and it felt like I hyperextended my back or something,” Tagovailoa told reporters. “It kind of hurt. I got up, and that’s kind of why I stumbled. My back kind of locked up on me. But for the most part, I’m good. Passed whatever concussion protocol they had.”

Per the NFL’s protocols, a player can be released for play following a concussion test if the team’s doctor and an independent neurologist determine that physical instability is not the result of a neurological issue. Typically, inquiries by the union stem from concerns about how a team doctor and independent neurologist might have come to the conclusion that a concussion isn’t the root cause of a player’s instability.

A league spokesperson said Sunday night that Tagovailoa underwent an examination by the Dolphins’ team doctor and an independent sideline neurologist and was cleared for return.