The NFL announced it will investigate to see if the proper concussion protocol was applied after Panthers quarterback Cam Newton was tackled in the season opener on Thursday against the Broncos.

The NFL said the inquiry does not suggest officials believe protocol was not properly followed but that it wants to protect the safety of the players.

The NFL Players Association said it will also launch a compliance investigation:

Despite several hard hits to the helmet in the NFL season opener, Newton was never taken out of the game to be evaluated by league medical staff.

Perhaps the most jarring of the hits came with seconds left to play in the game, when Denver Broncos safety Darian Stewart delivered a head-to-head blow.

The Broncos defeated the Panthers in Denver 21-20.

Newton told reporters staff evaluated him after the game.

"They asked me a couple questions, but nothing too serious," he said in a news conference Thursday.

In a statement, the NFL backed the medical staff in its decision to leave Newton on the field.

"There was communication between medical personnel on the Carolina sideline, including the unaffiliated neurotrauma consultant, and the two independent certified athletic trainer spotters in the booth.

"During stoppage in play while on-field officials were in the process of administrating penalties, the unaffiliated neurotrauma consultant and team physician requested video from the spotters and reviewed the play. They concluded there were no indications of a concussion that would require further evaluation and the removal of the player from the game."

Assessing a player for concussions is a collective effort that includes team physicians, athletic trainers, medical spotters who watch the game from a booth using binoculars or on video replay, and an unaffiliated neurological consultant, also known as a UNC, according to league protocol.

When asked if Newton's medical assessment was adequately handled, Panthers head coach Ron Rivera said he trusted the team's medical staff.

"I'm not going to question the doctors, OK," he said. “Anything that happens in the game that looks questionable, our doctor and the independent come together, they go and look at it immediately on instant replay and in this case, this happened."

The league protocol outlines additional triggers under which a player should be evaluated immediately, including a "big hit that is reasonably expected to give rise to a concussion."

Late Friday, a spokesman for the NFL Players Association told Channel 9 it had not decided if it would call for its own investigation on the decision to leave Newton on the field.

Violating the NFL's concussion rules could result in a fine of as much as $150,000 for the first offense.