This season for the New England Patriots has at times felt like a controlled experiment: Could the Patriots still play like the Patriots even if almost everything went wrong for them?

Tom Brady left for Tampa Bay. Eight of their players opted out of the pandemic season—more than any other team. New England’s two most important players, quarterback Cam Newton and cornerback Stephon Gilmore, contracted Covid-19.

But on Sunday the Patriots sunk to 2-5 on the season after an excruciating 24-21 loss to the division-leading Buffalo Bills, a defeat that further plunges the NFL’s pre-eminent modern dynasty out of the playoff picture and into unfamiliar territory. They’re closer to being one of the NFL’s worst teams than they are to being one of the best.

Despite all of the upheaval in New England, there was still a funny feeling the Patriots could still mechanically win football games. That’s because they have Bill Belichick, and it’s what they’ve always done. They haven’t finished below .500 since 2000, their last season before Brady became the team’s starting quarterback. Even when Brady missed most of the season in 2008, they went 11-5.

Sunday’s game was their best opportunity to vanquish the doubts that had emerged after the team’s sluggish start. They had a chance to beat the Buffalo Bills, who are leading the AFC East, and re-establish themselves in the picture for the NFL’s new seven-team playoff.

They were seconds away from doing just that. With under a minute left, they were marching down the field down 24-21. They were in an easy position to tie the game and send it into overtime. They were yards away from a game-winning touchdown.

Then Newton did the only thing he couldn’t in that scenario: he gave the ball away. He fumbled it on Buffalo’s 14-yard line, coughing up both the game—and the team’s chance at righting the ship.

It was an afternoon that put the franchise in foreign territory. No team has dominated its division the way New England has. The Patriots have won the AFC East in 17 of the last 19 years. The last time a different team claimed the title, Bills quarterback Josh Allen was 12 years old.

But at the moment, the one certainty in the NFL is now an uncertainty. That’s both because of the Patriots’ decline—and because after decades of mostly futility, someone else has established themselves as a legitimate threat. Actually, there are two someone elses.

Cam Newton of the New England Patriots scrambles against the Buffalo Bills.

Cam Newton of the New England Patriots scrambles against the Buffalo Bills.

Photo: Timothy T Ludwig/Getty Images

The first is the division-leading Bills, who are now 6-2. They made the playoffs as a wild card a year ago despite middling play from Allen, their young quarterback, and now he’s taken a dramatic leap in his third season that gives them a chance against anyone.

The second is the Miami Dolphins, who handed the reins to Tua Tagovailoa and dispatched the Los Angeles Rams 28-17. The most impressive part about the Dolphins’ win was that it happened despite such an unimpressive first start from the rookie quarterback. He passed for only 93 yards, but the defense forced four first-half turnovers and turned Jared Goff’s afternoon into a nightmare.

Both of these teams answered why they may be a force for years to come. The Patriots only left with more questions. They need to figure out if Cam Newton is their long-term—and short-term—answer at quarterback and more broadly if they have to engage in the type of full-scale rebuild that they have been able to avoid during their sustained run of success.

There’s always the chance they could pull off a stunning run this year. They lost in the final moments of this game after getting stopped in the final seconds against the Seattle Seahawks and Denver Broncos earlier in the year. They’re a 2-5 team that’s oddly close to being 5-2.

Belichick is still their coach. Newton made big plays in the second half before his big mistake. And next week, they have the ultimate cure for a four-game losing streak: they get to play the New York Jets.

Write to Andrew Beaton at andrew.beaton@wsj.com