BOSTON -- Two players violated league concussion protocol during last weekend's wild-card games, according to a letter sent by the NFL's head, neck and spine committee chairmen to all team doctors and trainers.

In a document obtained by The Associated Press, Drs. Hunt Batjer and Richard Ellenbogen said one player re-entered the game and another refused to leave the sideline.

The doctors did not identify the players, but one was Green Bay tackle David Bakhtiari, who went into the game for an extra-point try despite being examined for a concussion and not cleared. The other player was Saints cornerback Keenan Lewis, who remained on the sideline but did not get back on the field.

"On two occasions last weekend, and contrary to the advice of the team medical staffs, players who had been diagnosed with a concussion and therefore declared ineligible for play nonetheless refused to leave the sidelines as required by league concussion protocols," the letter said. "In one case, the player went back onto the field for one play before being removed from the game."

The doctors found "no fault" in how the team medical staffs conducted themselves.

"If a player refuses to follow your advice and leave the sidelines after being diagnosed with a concussion, we recommend that the head athletic trainer seek assistance from the player's position coach (or another member of the coaching staff) or from another team official to remove the player from the sidelines as soon as possible," the letter said.

The NFL's Madden Rule requires a player diagnosed with a concussion to be taken to the locker room or another quiet location.

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

The Saints and Packers declined comment. No fines will be imposed for the violations.

Bakhtiari's season is over because Green Bay lost to San Francisco. But Saints are playing Saturday at Seattle, and Lewis has been practicing.

Batjer and Ellenbogen noted in the letter how players may resist being kept out of the game, particularly during the postseason.

"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," they wrote.