The NFL has sent a letter to all team doctors after it was revealed that two players ignored the league's concussion rules during last week's wildcard playoff games.

Although the letter does not name those involved, ESPN reports that Green Bay Packers tackle David Bakhtiari and New Orleans Saints cornerback Keenan Lewis are the players in question.

The NFL players' association said: "We will continue working with the league to ensure that team doctors, coaches, trainers and other members of a team's medical staff enforce return-to-participation protocols.

"Players naturally want to play and ultimately, the game-day medical and coaching staffs have the responsibility and obligation for player protection and care.

"But [the rule] is an important element of the league's protocol and intended to safeguard the player's well-being and enhance his ability to recover from his injury."

League rules state that players should be taken to the locker room or a quiet location once diagnosed with a concussion.