In the latest step by the league to address a hot-button issue, commissioner Roger Goodell sent a memo to the 32 clubs Wednesday saying a player who gets a concussion should not return to action on the same day if he shows certain symptoms.
Those include an inability to remember assignments or plays, a gap in memory, persistent dizziness, and persistent headaches.
The old standard, established in 2007, said a player should not be allowed to return to the same game if he lost consciousness.
Wednesday's memo also says players "are to be encouraged to be candid with team medical staffs and fully disclose any signs or symptoms that may be associated with a concussion."
Nearly one-fifth of 160 NFL players surveyed by The Associated Press from Nov. 2-15 replied that they have hidden or played down the effects of a concussion.
The league said its concussion committee, team doctors, outside medical experts and the NFL Players Association developed the new standards.
NFLPA assistant executive director George Atallah said the union is "encouraged by this new policy." He added that the NFLPA "will continue to examine these issues independently to recommend the best possible policies and procedures."
The new policy states, in part: "Once removed for the duration of a practice or game, the player should not be considered for return-to-football activities until he is fully asymptotic, both at rest and after exertion, has a normal neurological examination, normal neuropsychological testing, and has been cleared to return by both his team physician(s) and the independent neurological consultant."
Teams were told this month they have to find an outside neurologist who can be consulted on concussions, and NFL spokesman Greg Aiello said Wednesday all of those independent doctors have been approved and are in place.
Since last month's congressional hearing on NFL head injuries, momentum has been building for changes in league policy. The revised return-to-play rules come about a week after Goodell sent a memo to clubs informing them that the two co-chairmen of the league's concussions committee had resigned and that he has been looking into possible rule changes.
On Wednesday, Cleveland Browns running back Jamal Lewis was put on injured reserve with post-concussion symptoms, ending his NFL career. Lewis had previously said he would retire at the end of the season. His teammate, starting safety Brodney Pool, also went on IR after getting at least his fourth known concussion last weekend.
They were among 11 players listed on Wednesday's league-wide injury report with concussions. Another eight were listed with head injuries.
The two starting quarterbacks from last season's Super Bowl - Pittsburgh's Ben Roethlisberger and Arizona's Kurt Warner - sat out last Sunday after saying during the week they planned to play despite getting head injuries the previous week.
"The evidence demonstrates that team medical staffs have been addressing concussions in an increasingly cautious and conservative way," Goodell wrote in Wednesday's memo. "This new return-to-play statement reinforces our commitment to advancing player safety. Along with improved equipment, better education, and rules changes designed to reduce impacts to the head, it will make our game safer for the men who play it, and set an important example for players at all levels of play."