From Week 7 of 2015 to Week 5 of 2017, the Chiefs went 27-4 in the regular season. Yeah, 27-4. From Week 6 to Week 11 of 2017, they went 1-4.
So what's gone wrong with Kansas City, particularly on defense?
During their 5-0 start, the Chiefs averaged 17.4 quarterback pressures per game, a rather high figure. That average includes a 26 quarterback-pressure performance in a Week Five win over the Texans and and 23-quarterback victory over the Eagles.
In their five most recent games – in which Kansas City has a 1-4 record – the Chiefs have mustered just 12.6 quarterback pressures per game. Their highest single-game quarterback pressure total in that span was an 18 in the 31-30 loss to the Raiders in Week 7.
Also, 2015 Defensive Rookie of the Year Marcus Peters hasn't played up to his normal standards during Kansas City's slide. In 2016, he finished with a +9.0 overall Pro Football Focus grade, the same as Patrick Peterson's. He received a +3.6 grade for his efforts in the first five games of his third year in the NFL. Since then, he's been given a +1.5 grade, although he has two picks in that time frame.
Peters is still one of the better cornerbacks in the league but hasn't been himself lately.
No Eric Berry has had a ripple effect on the Chiefs defense, obviously, and the lack of his run-stopping contributions have hurt Kansas City's ability to limit the opposition's ground game.
Last year, the perennial All-Pro safety played 1,085 snaps and finished with PFF's 11th-highest run-defense grade (+5.7) among 90 qualifying players at his position.
Unsurprisingly, the Chiefs haven't been able to reproduce his well-rounded production at the safety spot.
Strong safety Daniel Sorensen has the lowest run-stopping grade (-10.8) on Kansas City's defense, and cornerback-turned-free safety Ron Parker owns a -1.2 PFF run-defense grade.
At the beginning of the 2017 season, the Chiefs looked like a supercharged version of the clubs we saw in 2015 and 2016. But over the past five games, the offense has sputtered, and the defense has been stunningly porous, especially against the run. They've allowed 140.4 rushing yards per game at 4.61 yards per carry during their last five games.
Grades and stats courtesy of Pro Football Focus.