NFL Players Association spokesperson George Atallah announced on Twitter on Monday morning that the NFLPA is "initiating a full review" of the situation involving Houston Texans quarterback Tom Savage during Sunday's game against the San Francisco 49ers.

Savage left the game after being hit by Elvis Dumervil on an incompletion on third down. Video of the hit showed Savage lying on the ground for several seconds afterward as his arms trembled, and he appeared to exhibit what is known as a "fencing response," which has been cited as a visual sign of a concussion.

Savage was cleared to return for the following drive, which resulted in a three-and-out, before being ruled out because of a concussion.

NFL spokesman Brian McCarthy said on Sunday that Allen Sills, the league's chief medical officer, has already begun a standard review of Savage's injury, and will get reports from the spotters and the unaffiliated neurotrauma consultant.

Texans coach Bill O'Brien said in a press conference Monday that Savage was initially cleared by the medical staff. He was evaluated again after the following drive, not cleared and subsequently taken to the locker room.

"They said, 'We're going to check him again,'" O'Brien told reporters Monday. "I also said to (head athletic trainer Geoff Kaplan) at that point, our trainer, I said, 'Look, I think we need to check him again.' This was all going on at the same time. It wasn't anything particular that I saw on the field. I just said, 'Look, that was a quick check. It was a three-and-out on that series. Let's continue to check him to make sure that the player, Tom Savage, is OK.'"

O'Brien said he does not have access to video on the sideline, but if he had seen the clip that circulated on social media, "I would have never let that player back in the game."

The coach did not offer an update on Savage and said he expects backup T.J. Yates to start Sunday against the Jacksonville Jaguars.

"At no point in time is there anything more important to me than the safety of our players," O'Brien said. "I love our players. I care about them. And I cannot stand when players get injured. Again, with benefit of seeing the video that people are seeing, I would've never put him back in the game."

Contributing: Nancy Armour