Through three weeks of the regular season, the New England Patriots are in unfamiliar territory. With an average of 31.7 points per game against, the Patriots are dead last in the NFL in scoring defense. This is the same team that led the league in scoring defense a season ago at 15.6 points per game and has only finished with a below-average scoring defense once since Tom Brady took over as the starter.

The most head-scratching part of it all is that the Patriots’ defensive personnel is nearly identical to last season’s. The only major changes have been the loss of Chris Long, the injury to Dont’a Hightower and swapping out Logan Ryan for Stephon Gilmore.  Those three changes can’t realistically be responsible for such a drastic change in results. So, what then is responsible for the decline in performance of the Patriots defense, and can it be fixed?

To understand what’s going wrong with the New England defense, one has to first understand what makes it so unique. Bill Belichick is arguably the best week-to-week game-planner in the history of the sport, but those are usually only small wrinkles. There are certain schematic aspects at the core of the Patriots’ defense that are unwavering. The first of those has been the utilization of man coverage. Last season they ran some sort of man-to-man coverage at the seventh-highest rate in the NFL. While our data only goes back so far, they’d likely never be outside the top-10 in man rate with Belichick at the head.

The second big key is having their defensive line play run first and maintain gap control while rushing the passer. Staying run-first with the defensive line keeps them from getting gashed with a lineman getting too far upfield or a slant taking someone out of their gap. The gap control when rushing the passer is more personnel preference than anything else. It’s no surprise that Chandler Jones’s pressure rates went up drastically after leaving the Patriots for the Cardinals. In New England’s defense, rarely are the edge rushers given the freedom to simply attack the quarterback with reckless abandon. Jones was forced to rush the passer while also being wary of not losing contain. It forces opposing quarterbacks to win from the pocket while also rarely giving them room to step up and manipulate the pocket as four rushers close in around him.

The last key to their defense is trusting the coverage. They are not a blitz-heavy team. Belichick is not Bruce Arians in Arizona or Sean Payton in New Orleans, who have exclusively employed blitz-heavy defenses over multiple defensive coordinators. Last season they blitzed on a quarter of opposing teams’ dropbacks, the 25th-highest rate in the NFL. This season that number is 26.2 percent. In recent years, Belichick has taken that a step further, only rushing three on a good chunk of snaps. Last season they rushed three on 26.0 percent of dropbacks — more often than they blitzed and by far the highest figure in the league (6.7 percentage points more than second place). This season they’ve rushed three on 20.0 percent of dropbacks. Still an incredibly high rate.

When you put all three of those core tenets together, what do you get? Time for opposing quarterbacks. Offenses have figured out that when you play the Patriots, you’ll have time to operate downfield concepts. In fact, opposing offenses are averaging 3.1 seconds to throw against the Patriots, the highest figure in the NFL (league average is 2.7 seconds). And that number comes after facing Alex Smith and Drew Brees who were first and fourth, respectively, a season ago in terms of quickest time to attempt.

With all that time, opposing quarterbacks are shredding the Patriots’ defense. Through three weeks, opposing quarterbacks have attempted 54 passes coming 2.7 seconds or more after the snap. They’ve gained 544 yards on those passes. That’s over 10 yards per attempt. Cornerbacks, safeties, and linebackers can only stay in the hip pocket of a receiver for so long in man coverage before they allow some separation. If an offense can scheme knowing it will have time, they can bring a defense to its knees.

The bad news for New England is that it doesn’t look like it’s a problem that’s going to go away. The Patriots defensive line is simply not built to get quick pressure on opposing quarterbacks. The closest thing they have to a “speed” rusher is Cassius Marsh, and he ran a 4.89 40-yard dash at the combine. Deatrich Wise and Trey Flowers have graded out well as pass-rushers, but neither are true speed rushers off the edge. The Patriots have only been able to generate pressure on 32.3 percent of dropbacks this season, the 22nd-highest rate in the NFL (league average is 34.7). And that’s with quarterbacks sitting back there all day.

Unless Belichick wants to alter one of the core beliefs of his defense, they will struggle with this issue all season long. Getting Hightower and his dynamic pass-rushing ability back will help, but the issue will still persist. This secondary is still good enough to carry a weak front seven and 31.7 points per game is a bit of an aberration. They’ll get back to an above-average defense like they’ve always been under Belichick, but replicating last year’s results looks like a pipe dream at this point.

The blueprint is out on the Patriots defense so to speak, but with the NFL’s reigning MVP on the other side of the ball, they’re still the favorites in the AFC.

Mike Renner is a writer for Pro Football Focus and a contributor to The Washington Post’s NFL coverage.