The rivalry between the Buffalo Bills and Kansas City Chiefs has become a full-blown saga, with another chapter due in Sunday's AFC divisional round matchup at Highmark Stadium (6:30 p.m. ET, CBS). The teams will be playing for the seventh time this decade -- the series is split 3-3 over that time -- but it's the 2021 playoff matchup that has overshadowed them all.

It was Jan. 23, 2022 when Patrick Mahomes led the Chiefs to a 42-36 overtime win over Josh Allen and the Bills at Arrowhead Stadium, leading an improbable game-tying drive that started with 13 seconds remaining in the fourth quarter. Fans in Kansas City and Buffalo know the rest, but much has changed in the nearly two years since the event.

We took a look at the similarities and differences in a series that -- new faces and all -- should retain its inherent drama:

Patrick Mahomes carved out a major portion of his legend against the Bills on Jan. 23, 2022. Charlie Riedel/AP Photo

What has changed for the Chiefs since their epic playoff win over the Bills on Jan. 23, 2022?

The Chiefs turned over much of their defense in the two seasons since last meeting the Bills in the playoffs. Only four defensive starters from that game remain. Instead of a Chiefs defense stacked with veterans, the Bills will see a group that is the youngest in the NFL. Five defensive regulars for the Chiefs are in their second NFL seasons. On offense, the Chiefs no longer have Tyreek Hill and in fact all of their wide receivers except Mecole Hardman are new to the team since then. The major change on the coaching staff is that Eric Bieniemy, then the offensive coordinator, has moved on. He's been replaced by Matt Nagy. -- Adam Teicher

What has changed for the Bills since that game?

The Bills are on the team's second offensive coordinator change since that game, with Ken Dorsey hired to replace Brian Daboll in the 2022 offseason, and then Joe Brady hired as interim offensive coordinator when Dorsey was fired after Week 10 of this season. The change in playcaller to Brady has led to an increase in the team's designed rush percentage (up from 35.3% in 2021 to 40.8% in 2023), including a big rushing performance vs. the Cowboys.

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On the defensive side, coach Sean McDermott has taken over playcalling this season after Leslie Frazier left the team after the 2022 season.

Most of the key players are the same for the Bills, but there are new faces on the defensive side of the ball, including pass rushers Leonard Floyd and Von Miller and new linebackers with Tremaine Edmunds in Chicago and Matt Milano (right leg) out for the year. The Bills also added new offensive skill players through the draft in James Cook, Khalil Shakir and Dalton Kincaid. -- Alaina Getzenberg


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What looks the same for the Chiefs since the 2021 playoff win?

Patrick Mahomes and Travis Kelce. The last time the Bills played against the Chiefs without either one in the lineup? 2013. The Chiefs also have the same group of interior offensive linemen in center Creed Humphrey and guards Joe Thuney and Trey Smith. All are superior pass blockers. Only the four starters remain on defense, but each is a key player: tackle Chris Jones, linebackers Nick Bolton and Willie Gay and cornerback L'Jarius Sneed. Coordinator Steve Spagnuolo is back, as is much of the defensive coaching staff. Nagy replaced Bieniemy, but the offensive system and playcalling are still guided by Andy Reid. -- Teicher

What's the same about the Bills since that loss?

Allen and Stefon Diggs, to start. While the wide receiver has not had his normal production this season -- his last 100-yard receiving game was in Week 6 -- he remains someone to watch in this matchup, especially as wide receiver Gabe Davis continues to deal with a knee injury. Allen continues to be a threat as both a passer and a rusher behind an offensive line that has only seen change at the guard positions.

The other major familiarity for Mahomes is with safeties Jordan Poyer and Micah Hyde, who have been mainstays of the Bills' defense since 2017, and nickel corner Taron Johnson, who is progressing through concussion protocol. -- Getzenberg


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How has Patrick Mahomes evolved since the 2021 playoffs? Anything different about his game/approach?

Mahomes had in many ways the worst statistical season of his career. His QBR was a career-low 63, he had his fewest yards per game (261) and lowest yards per attempt (7.0). He threw more interceptions (14). He was affected by often sloppy play at wide receiver and on the offensive line, where penalties were a big problem. He was frustrated and even despondent at times when he felt let down by teammates after dropped passes or blocking issues. He went on an uncharacteristic rant after the Week 14 loss to the Bills when a late penalty cost the Chiefs a go-ahead touchdown. His game to an extent evolved with the Chiefs playing better defensively than they have since he became the starter. He became more content with throwing shorter passes. -- Teicher

How has Josh Allen's game evolved since the 2021 playoff loss?

Earlier in the season, this space would have been dedicated to Allen having fewer rushing attempts, but that number adjusted when Brady took over the playcalling. While Allen in general is sliding more as a runner and the number of hits he has taken overall are a bit fewer -- including a career-low with 24 sacks -- he also knows how to protect his body a bit better. He's always going to make the risky plays that either work or end in a turnover, but the Bills' effectiveness on the ground has adjusted Allen's game. That includes Allen connecting with running backs as receivers more often, which has increased his yards after catch per completion to 5.1, highest since 2018. -- Getzenberg

Beyond the above, what are the Chiefs doing better/worse than they did in the 2021 playoffs?

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The Chiefs aren't close to the high-scoring machine they were earlier in Mahomes' career. They finished 15th in the regular season in scoring. Offensive sloppiness was a season-long problem. The Chiefs dropped more passes than any other team and only one team had more offensive penalties. The Chiefs once routinely rallied for victories, but not so much this season. They lost five games by one score and in four, the Chiefs had the ball late with a chance to win or send it into overtime. They failed to score each time. But the Chiefs are stronger defensively. They were second in the league in points allowed and sacks. They had two players (George Karlaftis and Jones) with more than 10 sacks for the first time since 2018. -- Teicher

What are the Bills doing better/worse than they did in the 2021 playoffs?

This isn't the same Bills defense that gave up a field goal drive with 13 seconds left two years ago. Many of the faces are different, but beyond that, this is a unit that has seen an uptick in overall yardage allowed over the course of seasons since 2021 (from 289.2 yards per game to 308.1 this season) while creating more sacks (6.7% of sacks per dropback in 2021 to 8%). The Bills' defense is blitzing less since 2021 (24.7% to 21.1%).

One area harder to quantify is the Bills' ability to adjust to the loss of injured players. Losing cornerback Tre'Davious White for the end of 2021 was significant, but this year as injuries have continued to pile up, the Bills have been able to adjust and shown off depth. -- Getzenberg