The Kansas City Defense Is Looking Like Its Old Self. That Could Be Bad News.

When the Kansas City Chiefs beat the Buffalo Bills on Sunday, they won the Super Bowl — at least in the minds of some prominent analysts. With the NFC’s top three seeds already out of the playoffs, and the fourth-seeded Cincinnati Bengals the last remaining AFC contender, the No. 2-seed Chiefs are the strongest team left standing. FiveThirtyEight’s Elo predictions make the Chiefs a 10-point favorite over the Cincinnati Bengals in the AFC championship game and give Kansas City easily the best chance to win the Super Bowl. 

The Chiefs don’t have to worry about the team that beat them in the last Super Bowl because, in the divisional round, the Tampa Bay Buccaneers ran into a team whose strengths lined up directly with their weaknesses. And in fact, Kansas City should be worried about a similar fate as the Bucs: The Bengals’ explosive passing game might blow right through a Chiefs defense that has regressed to its terrible early season form.

After the Chiefs lost to the Tennessee Titans in Week 7, they flipped a remarkable defensive switch. They retooled their secondary to use more defensive backs more effectively, traded for pass-rusher Melvin Ingram and transformed one of the worst defenses in the league into one of the best.

But then the Bengals hung 475 yards and 34 points on them in their Week 17 matchup, and now it looks like the Chiefs’ defensive “good switch” has flipped … back:

Kansas City’s defense played at as high a level as any in the NFL for nine weeks, but over the past four, the Chiefs’ per-game defensive stats have fallen back to their beginning-of-the-season levels. Though those games included matchups against the high-scoring Bengals and Bills, the other two were against bottom-half offensive squads in the Denver Broncos and Pittsburgh Steelers.

What happened? For whatever reason — injuries to key players, strengths of their opponent, something else entirely — Chiefs defensive coordinator Steve Spagnuolo got away from what had been working so well.

According to ESPN’s Stats & Information Group, there’s a slight positive correlation¹ between the Chiefs’ defensive production (in expected points added per play) and how often they play dime-package defense. Most of their best per-snap defensive performances came in games that saw six defensive backs on the field at least 30 percent of the time, and most of their worst came when that package was employed less often:

The Chiefs started out the season regularly playing a smaller share of dime coverage before bumping it up in the middle of the season. But after discovering this apparent “good switch” for his defense, Spagnuolo reverted back to his early season tendencies. Their games in Weeks 15 and 16 featured two of their four lowest dime-package rates of the year, per ESPN Stats & Information Group. And in Weeks 17 and 18, their dime rates inched up some but stayed under 30 percent — and their defensive EPA per play was below zero.

In some situations, Spagnuolo’s hand has been forced. Cornerback Rashad Fenton, the Chiefs defensive back with the highest Pro Football Focus overall (82.0) and coverage (82.1) grades, has been in and out of the lineup with injuries and COVID-19; due to a back injury, he hasn’t played since Week 18. Veteran safety Tyrann Mathieu suffered a first-quarter concussion against the Bills and had also showed up on the injury report ahead of the Week 17 Cincinnati game (quadriceps). And slot corner L’Jarius Sneed has been playing through a knee injury since Week 18.

In that first Bengals game, in which the Chiefs played dime on 25.4 percent of snaps, Sneed (67.2) and Fenton (61.2) had the best coverage grades of a terribly graded Chiefs secondary, and no other member cracked 60.0. The other starting outside corner, Charvarius Ward, turned in a miserable 35.1; much-maligned strong safety Daniel Sorensen was given a brutal 28.2. 

The Chiefs’ grades for that game were depressed across the board at least partially because Cincinnati deployed rookie standout Ja’Marr Chase all over the field. Seven different defenders were credited with defending him on at least one target; he caught 11 of 12 targets for a rookie-record 266 yards and three touchdowns. Sorensen was credited with only one coverage matchup against Chase, but it was a notable one:

Sorensen’s transition from combination safety and slot corner to special-teamer and rotational box defender was one of the changes that sparked the Chiefs’ defensive turnaround; he averaged only 10.9 coverage snaps from an outside-the-box position² from Week 6 to Week 13. But he’s been pressed into duty more often in latter weeks, and his 23 defensive-back coverage snaps against the Bills were the third-most he had played all year.

In that game, the Chiefs earned their second-lowest PFF team coverage grade of the season (46.5). Their fifth-lowest (49.9) came in Week 18, against the Broncos. And for all their brutal early season performances, Kansas City’s lowest team coverage grade all year (45.7) came in Week 17, against the Bengals. If nobody on Kansas City’s roster could cover Chase four weeks ago, it’s unlikely that will change this weekend.

Fenton practiced in full on Wednesday and should be able to go against the Bengals. Mathieu thinks he’s “got this,” in terms of being ready to play — but the NFL’s concussion protocol is a five-stage process with physician oversight and approval.

If either can’t go, let alone both, Spagnuolo and the Chiefs will have to rely on Sorensen for more coverage snaps or use their dime package less often, or some combination thereof. That would keep the Chiefs’ defensive “good switch” pushed in the wrong direction as they again match up against an offense they have struggled to contain.

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