"Promptly after the conclusion of yesterday's game, we began a review to determine the facts of the injury to St. Louis quarterback Case Keenum and why he was not removed from the game for the necessary evaluation by a team physician or the unaffiliated neuro-trauma consultant as required by our concussion protocols," the league said in a statement. "We are continuing that review today, which includes discussions with the Rams and their medical staff, the ATC spotter [a certified athletic trainer], the game officials, our medical advisors and the NFLPA [NFL Players Association].
"In the meantime, prior to this week's games, we will reinforce with all involved the need to ensure that these injuries are properly identified and addressed in a manner consistent with our protocols."
NFLPA director George Atallah said the players' union is also investigating the matter.
The NFL and NFLPA will lead a mandatory conference call for the league's 32 head athletic trainers on Tuesday, league and union sources told ESPN's Chris Mortensen. Team doctors have been invited to participate in the call, which will discuss the Keenum incident and further review concussion protocols.
With 1 minute, 10 seconds left in Sunday's game, Baltimore linebacker Elvis Dumervil jumped offside. The play was not blown dead, and Ravens defensive end Timmy Jernigan eventually broke through and tackled Keenum, who landed hard as the back of his head bounced off the ground.
As the officials sorted out the penalty, Rams head athletic trainer Reggie Scott came on the field to talk to Keenum while backup quarterback Nick Foles warmed up on the sideline.
But Keenum stayed in the game for the next two downs, including a third-down fumble that led to Baltimore's game-winning field goal.
After the game, Rams coach Jeff Fisher didn't seem to know that Keenum had suffered a concussion. Asked if he considered taking Keenum out of the game, Fisher said no.
A moment later, Fisher was asked what his offense needs to do to improve, and he didn't sound like someone who had a quarterback about to enter the concussion protocol.
"We'll give Case a good week of practice, and we'll expect him to do better than he did today," Fisher said. "You can see he's mobile, he can move around, he can do things."
The Rams announced Keenum's concussion diagnosis after Fisher's news conference, about 30 minutes after the game ended.
The NFL added independent certified athletic trainers in 2011 with the intent to have them serve as spotters from the press box. A video component was added during the 2012 playoffs.
Starting this year, those spotters have been given the ability to buzz the officials for medical timeouts if a player appeared to need medical help. Those spotters' duties include an emphasis on head injuries and concussions but are meant to span all potential injuries.
Keenum wasn't the only quarterback in Sunday's game to keep playing with an injury. Baltimore's Joe Flacco finished the game with a torn ACL.
NFL investigating Keenum concussion
Chris Mortensen breaks down the series of events that led to Rams QB Case Keenum remaining in the game after appearing to sustain a concussion and the league's investigation into why protocol was not followed.