Through the first half of the 2019 season, it looked like the Chiefs were going to have to take the same trip to the postseason they had in 2018 — riding Patrick Mahomes all the way, and hoping their defense didn’t blow it all up. The last straw for Andy Reid and the Chiefs’ front office for ex-defensive coordiantor Bob Sutton was Kansas City’s loss to the Patriots in the 2018 AFC Championship game, when the Chiefs never got the ball in overtime, and New England marched down the field on a 13-play, 75-yard touchdown drive. Sutton’s defense was helpless to stop it, and things had to change.

To replace Sutton, the Chiefs hired Steve Spagnuolo, the architect of the 2007 Giants defense that stunned the Patriots in Super Bowl XLII. Spagnuolo’s multiple fronts and aggressive coverages provided a different template than Sutton’s far more passive approach, and though it took a while for everything to gel, the Chiefs now have a championship-caliber defense at exactly the right time.

Through the season’s first nine weeks, Kansas City allowed 14 touchdowns and had just six interceptions, though their seven dropped interceptions (tied with Cincinnati for the league lead) was a precursor of better things to come. Kansas City allowed 7.02 yards per attempt, and an opposing QBR of 90.50. Not the worst in the league in any of these departments, but hardly the kind of defense Spagnuolo or Reid wanted. Their opponent completion rate of 62.96% ranked 11th-best in the league.

Rex Burkhead’s game-winning touchdown run in the 2019 AFC Championship game spelled the end of Bob Sutton’s tenure. (Jay Biggerstaff-USA TODAY Sports)

But from Weeks 10-17, the change has been remarkable. The Chiefs are tied with the Ravens for the fewest passing touchdowns allowed with seven, and have picked off 10 passes — tied with the Falcons, Browns, Colts, Saints, and Dolphins for the most in that span. They also have seven dropped picks in the second half of the season Only the Steelers, Packers, and Ravens have allowed a lower completion percentage than Kansas City’s 57.36%.

The Chiefs have allowed 6.22 yards per attempt — only the Ravens, 49ers, and Steelers have been better, and only the Steelers have allowed a lower QBR than Kansas City’s 68.72. The Chiefs’ defensive Positive Play Rate (the percentage of plays in which an opposing offense has Expected Points Added over zero) has dropped from 46% to 42%.

Perhaps most importantly for their playoff prospects, the Chiefs have become the kind of team that can beat other postseason entrants without a superhuman effort from Mahomes — or even with Mahomes in the game. Playing through injuries as he has, Mahomes saw a drastic dip in production from his 2018 MVP season, but the Chiefs beat the Vikings, 26-23 in Week 9 with Mahomes on the bench and Matt Moore throwing for 275 yards and a touchdown. They lost a close one, 31-24, to the Packers the week before with Moore as their quarterback. They beat the Patriots, 23-16 in Week 14, with Mahomes challenged by New England’s top pass defense, managing 283 yards and a touchdown. And now, they have a revenge match on Sunday in the divisional round against the Texans, who beat them 31-24 in Week 6, back when this defense wasn’t what it would become.

The unquestionable key to this turnaround, besides everyone getting on the same page with Spagnuolo’s more aggressive man and match concepts, has been defensive back Tyrann Mathieu, the former Cardinals and Texans star who signed a three-year, $42 million contract with the Chiefs in March. As has been the case through his career, Mathieu is a game-changer from every possible position. This season, he’s played 1,080 snaps — 82 at the defensive line, 315 as a box safety or nickel/dime linebacker, 483 as a slot cornerback, 27 as an outside cornerback, and 173 as a free safety.

“He wears a lot of hats,” defensive backs coach Sam Madison recently said, via ESPN’s Adam Teicher. “Early in the year, we just wanted to put guys in situations to be comfortable. He understands the defense. … Since he’s been at LSU, he’s played all over the place. It was just a natural fit for him. He’s taken to it very well. I talked to him a few weeks ago, telling him just to be patient and try not to go out there and find the plays [but] let them come to him. That’s what he’s been doing.”

Mathieu’s transformation in Spagnuolo’s schemes mirrors the overall defensive improvement.

“He’s the glue,” Spagnuolo has said of Mathieu. “He’s the guy that can get in the huddle in practice and say, ‘Hey, we need to step it up.’ You need guys like that. That stuff, I value as a coordinator. It makes my job a lot easier.”

As a slot defender through the first half of the season, he allowed 16 receptions on 21 targets for 132 yards, a touchdown, no interceptions, and an opponent passer rating of 107.6. From Week 10 on, he increased his efficiency with 17 catches allowed on 27 slot targets for 110 yards, no touchdowns, and an opponent passer rating of 40.2.

As a safety, his coverage improvement has been similarly transformative. In the first half of the season as a box or deep safety, he allowed 23 catches on 34 targets for 221 yards, two touchdowns, one interception, and an opponent passer rating of 92.9. In the second half of the season, he allowed 28 completions on 42 targets for 193 yards, no touchdowns, three interceptions, and an opponent passer rating of 47.0.

Chiefs head coach Andy Reid knows his safeties — he had Eric Berry from 2013 through 2018 in Kansas City, and Hall-of-Famer Brian Dawkins in Philadelphia from 1999 through 2008. As far as Reid is concerned, Mathieu belongs in that discussion.

“This organization was blessed to have Eric Berry… a great leader, tough, all those things. Before that I was with Brian Dawkins. I’ve been lucky to be around some really good safeties. They’re all different in their own way but they’re all great football players with great instincts and good leaders. They lead a different way. This kid here, he’s a special kid. He’s not the biggest guy. You’re not looking at one of those huge safeties but, man, is he a good football player. He just gets it.”

Leadership is one thing. The 5-foot-9, 190-pound Mathieu also brings top-tier athleticism, field awareness, and the ability to read a play and snuff it right out — from anywhere on the field.