The N.F.L. was facing one of its worst crises in decades as Buffalo Bills defender Damar Hamlin remained in critical condition on Tuesday after collapsing during a prime-time game in Cincinnati, raising fresh questions about ever-present serious injury in America’s biggest sport.

Hamlin, 24, collapsed in the first quarter of a highly anticipated matchup with the Bengals on Monday night, forcing the league to suspend the game. As Hamlin lay on the field motionless, many of his teammates in tears nearby, doctors pumped his heart in a suddenly hushed stadium.

After Hamlin’s heartbeat was restored and he was taken off the field by ambulance, to be transported to a hospital trauma unit, the coaches conferred with Shawn Smith, the head referee, and the players walked into their locker rooms. About 30 minutes later, the league formally postponed the game, and the Bills later flew back to Buffalo.

The dark turn was a reminder that the N.F.L. has become America’s most popular league despite an always-present risk of violence. With the regular season winding down and the playoffs around the corner, the league has seen a record number of close contests and jaw-dropping plays, and has been richly rewarded by broadcasters and sponsors for them.

But the N.F.L. juggernaut flipped to a prime-time nightmare that overtook a national showcase between two championship contenders. The question of violence that always hovers over N.F.L. contests had once again rocked the league. Hamlin’s cardiac arrest was no torn knee or busted ankle. It was potentially life ending, the most frightening type of injury in a sport filled built on frightening collisions.

The reaction from fans and football veterans was swift, and both predictable and confusing. There were many expressions of support. Sensing the gravity of the situation, many other N.F.L. teams sent well wishes to Hamlin on Twitter. Millions of dollars were donated overnight to a fund-raiser that Hamlin had set up to pay for toys to be distributed at a day care center run by his mother, Nina.

At the same time, television viewers heard Joe Buck, ESPN’s play-by-play broadcaster for the game, say that the players, just before they returned to the locker rooms, were told they would have about five minutes to get ready to play again. Bengals quarterback Joe Burrow could be seen tossing a football.

“That’s the word we get from the league and the word we get from down on the field, but nobody’s moving,” Buck said.

In a news conference about three hours later, the N.F.L. denied that there was any consideration to restarting the game.

“Immediately, my player hat went on,” Troy Vincent, the N.F.L.’s executive vice president of football operations and a former cornerback, said to reporters. “How do you resume play after you’ve seen such a traumatic event occur in front of you real time? And that’s the way we were thinking about it.”

Whatever the truth, football fans — and even former star players — are once again asking whether the game they enjoy is worth the risk. Ryan Clark, a former hard-hitting defensive back who is now an analyst on ESPN, said many players fool themselves into thinking they are modern-day gladiators when in fact they are highly paid entertainers smashing their bodies for a living.

“We use these cliches. ‘Going to war,’ ‘willing to die,’ ‘give it all,’” Clark wrote on Twitter on Monday night. “That’s all talk. It’s a game. A game! You never suit up & think you’re not going to make it home.”

Hamlin’s collapse was far from the only reminder of football’s “next man up” culture in a league where the lack of guaranteed contracts incentivize players to return to action as quickly as possible. Indeed, Hamlin had joined the Bills’ starting lineup in September as a replacement for Bills safety Micah Hyde, who has been out with a neck injury.

On Sunday, Indianapolis Colts quarterback Nick Foles left a game after being sacked by the Giants linebacker Kayvon Thibodeaux, who celebrated the hit as Foles appeared to convulse on the field. Foles was carted off but was listed with a rib injury.

Also on Sunday, Philadelphia Eagles defensive end Josh Sweat left the field on a cart after trying to make a headfirst tackle on New Orleans Saints fullback Adam Prentice. Sweat stayed on the ground facedown for several minutes and raised his arm to the crowd as he left the field. He later vowed on Twitter to return this season.

In September, Bills cornerback Dane Jackson injured his neck and was immobilized and transported in an ambulance to a hospital in Buffalo. He was released from the hospital the next day and returned to play in October.

Ten days later, Miami Dolphins quarterback Tua Tagovailoa was taken to a hospital after slamming his head against the turf in a game against the Bengals. Tagovailoa immediately raised his hands and his fingers were splayed, a gesture called a “fencing response” that can be a sign of brain injury.

The concussion highlighted an investigation that began the prior week into how the Dolphins responded after Tagovailoa appeared to be concussed in a game against the Bills. The league subsequently updated its concussion protocol to prohibit a player from returning to play if he shows ataxia, a term describing impaired balance or coordination caused by damage to the brain or nerves.

Tagovailoa was again diagnosed with a concussion after being sacked on Dec. 25 in a game against the Green Bay Packers.

Injuries and even deaths are not uncommon in football. Every year, a handful of high school football players die, some from heat stroke, some from broken necks. Families and communities are shattered. Yet while participation in high school football has slipped in recent years, it remains the most popular sport among boys.

The N.F.L. is another realm because it has turned the game into mass entertainment, complete with cheerleaders, packed stadiums and big-name sponsors hawking their products. Yet the N.F.L. knows the game’s violence has turned off fans, and has watched families steer their sons into baseball, basketball and soccer.

So the league takes pains to remind fans that it is using its vast resources to “make the game safer” and “take the head out of the game.” In 2019, the league even produced a video on how to recognize and rescue players who suffer sudden cardiac arrest.

But tackle football centers on players crashing into one another on every down, and no amount of dollars, training and good intentions will change that. The best the N.F.L. can do is reduce risk, not eliminate it.