Buffalo Bills head coach Sean McDermott watches from the sidelines during the first half against the Kansas City Chiefs on Sunday in Orchard Park. AP photo

ORCHARD PARK — Just when it appeared fashionable in some NFL circles to begin writing off Josh Allen and the Bills as mere pretenders, Buffalo went ahead and flipped the script.

The Bills reached the midpoint of their season at 6-2 with what stands as a signature 28-21 victory over nemesis Patrick Mahomes and the Kansas City Chiefs on Sunday.

Impressive as the outcome was for Buffalo, which entered as 2 1/2-point underdogs, more encouraging was how the offense and defense played complementary parts in finishing off the Chiefs.

The Bills put aside questions about how they padded their record with a soft season-opening schedule. Buffalo’s first five wins came against opponents entering Week 10 with a combined 12-31 record. Meanwhile, their losses came against New England (7-2) and Atlanta (3-5).

The criticisms didn’t stop there.

Buffalo’s offense appeared out of sync, with receivers showing an inability to separate in coverage. And a young and injury-depleted defense was prone to inconsistency against the run — Atlanta burned it for 210 yards rushing — and the pass in Stefon Diggs’ 10-catch, 146-yard outing for New England.

None of those issues arose on Sunday.

Allen was nearly perfect, going 23 of 26 for 273 yards to set the franchise record for best completion percentage with 15 or more attempts. And he managed to go deep several times, starting with a 23-yard game-opening TD pass to Dalton Kincaid.

The defense, meanwhile, was relentless in harassing Mahomes, who posted one of the worst statistical days of his nine-year NFL career. Going 15 of 24 with an interception, the two-time NFL MVP failed to complete 50% of his pass attempts for the first time, and Mahomes’ 57.2 passer rating was his second lowest.

Though it’s only one game, it was an outing in which the Bills began showing signs of forming an identity.

The offense displayed an ideal balance, with James Cook gaining 114 yards rushing. And the defense was on the attack, not allowing Mahomes to settle in the pocket. He was sacked three times and hit 15 times overall.

Then again, the Bills’ evolution was supposed to be a work in progress all along.

Coordinator Joe Brady reminded critics that a year ago, the offense didn’t begin gelling until a 31-10 win over Seattle in Week 8.

The defense, meanwhile, has featured a revolving door of players because of injuries, suspensions and youth, making it difficult to establish any semblance of continuity.

More changes are coming with safety Taylor Rapp (knee) and defensive end Michael Hoecht (right Achilles tendon) expected to miss the rest of the season.

The loss of Hoecht is substantial. After missing six games while serving an NFL suspension for violating the league’s performance-enhancing drugs policy, the do-it-all defensive lineman’s presence was notable in the two outings he played before getting hurt.

There remain challenges ahead for the five-time defending AFC East champions, with games against Tampa Bay, Pittsburgh, Philadelphia and a rematch against New England looming.

And beating the Chiefs in the regular season for a fifth straight meeting since 2021 doesn’t make up for Buffalo going 0-4 in the playoffs against Kansas City dating to 2020.

Coach Sean McDermott said as much when closing his news conference on Sunday.

“We’ve got to move forward,” he said, looking ahead to a trip to Miami on Sunday. “Wake up tomorrow, turn the page and move on to the Dolphins.”

WHAT’S WORKING

Running attack. Cook has topped 100 yards rushing five times. He became the first player to do so against Kansas City since Baltimore quarterback Lamar Jackson had 122 yards rushing in the 2024 season opener.

WHAT NEEDS HELP

Defensive line. Hoecht’s injury comes with starting DT Ed Oliver expected to miss at least a month with a torn left bicep. Meanwhile, fellow starting DT DaQuan Jones has missed three games with a calf injury.

STOCK UP

Kincaid. The third-year tight end has proven to be Buffalo’s most consistent receiving threat. He leads the team with 411 yards receiving and four TD catches.

STOCK DOWN

K Matt Prater. After Prater hit 12 of his first 13 field-goal attempts, his 52-yard miss off the right upright prevented the Bills from building a two-score lead with 27 seconds left. Reliable as Prater has been filling in for injured Tyler Bass (hip/groin), another miss could lead to Buffalo considering activating Bass off IR.

INJURIES

Starting nickel cornerback Taron Johnson is considered week to week after hurting his groin in practice last week.

KEY NUMBER

6-1 — The Bills’ record this season when scoring a TD on their opening drive. And they’re 13-2 when doing so dating to the start of last season, including playoffs.

NEXT STEPS

Travel to face division rival Miami (2-7) on Sunday. The Bills have won seven straight meetings and 14 of the past 15, including playoffs and a 31-21 home win in Week 3.