MIAMI - Three teams begin preparation for games this weekend unsure if their starting quarterback will be cleared to play as the league's stricter concussion rules start to bite.
The NFL has been concerned about the health implications from concussions and brain injuries and introduced a clearly defined series of steps that teams have to follow when players suffer knocks to the head.
In order to return to action the following week, concussed players have to be cleared by an independent neurologist.
The tougher stance from the NFL came against the backdrop of legal action brought by former players, who suffered what they say were long term effects of concussions, and a Congressional hearing on head injuries in American football.
An illustration of the new rules is none more evident than next Monday's game between the Chicago Bears and San Francisco 49ers, which could now feature two back-up quarterbacks after both teams lost their starters on Sunday to head injuries.
Jay Cutler of the Bears was forced to exit Sunday's 13-6 defeat to the Houston Texans at half-time after showing symptoms of concussion following a helmet-to-helmet hit from Houston's Tim Dobbins.
Cutler came back in after the hit, taking seven more snaps before the Bears said concussion-like symptoms became evident.
Several reports said the NFL had reviewed the Bears' handling of the situation and agreed they had followed the protocol correctly.
But Bears head coach Lovie Smith said that even without the rules he would not have sent a dizzy Cutler back on the field.
"If a player has a concussion or any injury he's not going back in the game. It's as simple as that," said Smith.
"We'll never put a guy at risk. No game is that important for us. The players' health always comes first with us," added Smith.
Jason Campbell, who completed 11 of 19 passes, putting up 94 yards in tricky, wet conditions, will start if Cutler is not ready.
"It's tough but that's why you bring in a veteran like Jason Campbell who has played ball," smith said.
"In the NFL injuries happen and you have to be ready. Jason is a pro," he said, adding that the eight-year veteran usually only took a quarter of the snaps in practice.
San Francisco quarterback Alex Smith also went out with concussion during the 24-24 tie with the St. Louis Rams on Sunday after taking a couple of hits.
Smith complained of blurry vision and was replaced by second-year Colin Kaepernick who produced 11 of 17 for 117 yards with 66 rushing yards and a seven yard touchdown run.
"I thought he handled himself pretty well, overall I thought he had a solid performance," 49ers head coach Jim Harbaugh said of Kaepernick.
It is a sign of how seriously teams take the rules and the threat of sanction that both have been at pains to point out while their players recovered to take more snaps, coaching and medical staff followed the rules in bringing a player out once concussion symptoms were identified.
Symptoms included in the NFL's rules include a gap in memory, persistent dizziness, headaches and an inability to remember assignments or plays.
Previously the NFL's rules stopped a player from returning to action on the same day only if he lost consciousness.
In the third case, Philadelphia Eagles' Michael Vick suffered what his coach Andy Reid called on Monday a "pretty significant concussion" in the defeat to the Dallas Cowboys and said the player's memory of what happened was "foggy".
Vick was told to rest at home after suffering the injury in the second quarter and Reid may well have to turn to back-up Nick Foles for his first start against the Washington Redskins. (Editing by Greg Stutchbury)