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Defenses Are Slowing Down Great Passing Attacks. Your Move, Chiefs (And Bills).

In this space a week ago, I detailed how Patrick Mahomes of the Kansas City Chiefs is enduring the first bona fide slump of his entire career. Up until Week 8 of this season, Mahomes had gone an incredible 61 straight starts to begin his career without stringing together so much as two back-to-back below-average outings, according to our Elo QB value metric. But now, Mahomes has been subpar for three consecutive games, after averaging just 4.2 net yards per attempt — and leading the Chiefs offense to just 13 points — on Sunday against the Green Bay Packers.

Mahomes isn’t the only typically electrifying QB looking for answers this week. Josh Allen of the Buffalo Bills isn’t necessarily as deep into his slump as Mahomes is, but the Bills’ ugly 9-6 loss to the woeful Jacksonville Jaguars (in which Allen had 4.5 net yards per attempt, three turnovers and zero touchdowns) is sounding alarm bells for a team that, like Kansas City, had once looked like the leading contender in the AFC.

Mahomes and Allen might want to blame their problems on the same source: the strategies of opposing defenses. The Chiefs have been beguiled by two-high-safety coverages for most of the season, while the Jags played double-high on 41 of Buffalo’s 51 pass plays Sunday, the most plays on which any team used that pass defense all weekend. “Just two-high shells, forcing us to throw underneath,” Allen said when asked what Jacksonville had done to hold the Bills to a pair of field goals. But even if Kansas City and Buffalo know what the problem is, can they craft a solution on the fly after building in a certain way for years?

As Joe Buscaglia of The Athletic recently noted, this is all part of a pattern of high-flying offenses getting what he calls the “Chiefs treatment” from opposing defenses. According to ESPN’s Stats & Information Group, Kansas City has faced two or more high safeties on a league-high 82 percent of its pass plays this season, up from 37 percent last season. Buffalo is third on the season, at 62 percent, but that number is up to 68 percent in the past two games, including a Chiefs-like 80 percent Sunday. Coverages that keep at least two defensive backs over the top in zone support are good for neutralizing deep shots down the field, taking away some of the most dangerous weapons in the arsenals of the strong-armed Mahomes and Allen.

Two-deep coverages can be beaten with certain route combinations that force the safeties to respect a vertical threat and leave soft spots underneath. The Chiefs and Bills have been trying to take what those defenses are giving them, which is why Mahomes and Allen have been held to so few air yards per attempt in recent weeks. (Mahomes ranks 17th among qualified passers since Week 7, and Allen ranks 28th; both were among the top half of QBs last year.) But it’s frustrating for fans of two of the game’s most gifted arms to watch them settle again and again for short throws when everybody — especially the defense — knows they’d much rather be heaving deep bombs.

The other thing that can force a defense away from playing so many two-high shells is a strong running attack. Defenses show their respect for opposing rushing offenses by bringing defenders up into the box, nearer to the line of scrimmage, to give them greater numbers to match up with blockers and ultimately slow down the run. But more coverage-oriented schemes sacrifice run defense to stop the pass. Against two-high-safety looks, NFL teams are gaining 4.9 yards per carry this season, a whole yard more than they gain against all other types of defenses combined (3.9 yards per carry). If a team can punish opposing defenses by running the ball, they’ll bring more defenders into the box and open up more chances to throw downfield.

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Trouble is, the Chiefs and Bills are both below-average rushing teams, according to schedule-adjusted expected points added (EPA). Even as it was going to its second straight Super Bowl last season, K.C. ranked 17th in rushing EPA on offense; that ranking sits at No. 18 so far this season. Buffalo is even less balanced between its rushing and passing — it ranked 24th in rushing EPA last season and has fallen to 26th this year. This type of team construction actually makes sense by the numbers: Across the history of modern football, particularly since the aerial explosion of the 21st century, passing efficiency is far and away more correlated with scoring success than rushing efficiency. A new class of more analytically minded coaches and front offices responded to that fact by gearing game plans toward passing and stopping the pass … though Kansas City and Buffalo’s recent struggles may be showing the limits of neglecting rushing offense as a matter of strategy.

Of course, maybe all any team getting the “Chiefs treatment” needs to do is simply resolve to run more against those two-high looks. K.C. is averaging 5.5 yards per carry this season when opponents use at least two high safeties, according to ESPN’s Stats & Info, and Buffalo is averaging 5.3 — both numbers much higher than their overall rushing averages of 4.6 and 4.4 yards per carry, respectively. Although the leaguewide numbers are somewhat deceptive because defenses traditionally tend to employ two-high coverages more in obvious passing situations — where a 5-yard chunk often does an offense little good — the Chiefs (and, increasingly, other teams) are starting to face double-high on more than three quarters of their offensive snaps. At those rates, situational considerations go out the window: Opponents are practically begging the offense to run all over them.

But will the Chiefs and Bills oblige? The answer lies in the specifics of how each coaching staff feels it can best counterattack the looks defenses are throwing at it. Kansas City has had weeks of abnormally mortal offense to analyze, though it hasn’t seemed to find a consistent answer yet. (Don’t bet on Andy Reid lacking one of those indefinitely.) Buffalo recognized it was being played in a similar way, so we’ll see how Allen and company come out in response. The chess match between defenses and these talented passing attacks was badly tilted toward the offenses over the past few years; now that things have turned some in the opposite direction, we might get to see yet another tactical evolution in the endless struggle between the two sides of the ball.

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Neil Paine was the acting sports editor at FiveThirtyEight.