The NFL and NFLPA have initiated a standard review of concussion protocols after New York Giants receiver Sterling Shepard suffered a concussion last Sunday and was allowed to finish the game, league sources told ESPN on Saturday.

There was a breach in the protocol and now both sides are trying to figure out how and why, sources told ESPN.

The controversial play occurred in the third quarter of Sunday's game against the Dallas Cowboys, when Shepard ran into cornerback Anthony Brown and fell backward to the ground, clearly staggered.

Shepard struggled to get up and waved off the Giants' calls to come out of the game.

The NFL deploys neurological spotters on both sidelines -- as well as another set of eyes in the press box -- to prevent players from remaining in the game after they are knocked woozy. However, Shepard somehow evaded all NFL officials -- and the Giants staff -- to not miss a play and finish the game.

Over the past two seasons, just three plays out of 1,200 concussion evaluations were reviewed. All three plays happened during the 2017 season, and they involved Russell Wilson, Tom Savage and Cam Newton.

This incident involving Shepard is reminiscent of the last time there was this much attention brought to a hit -- in December 2017, when Savage, then with the Houston Texans, took a hit on third down that left him on the ground with his hands twitching. After being checked in the sideline medical tent, Savage was allowed to re-enter the game, and controversy ensued.