We expected the New England Patriots’ defense to rank among the very best in the NFL this season. They’re currently the league’s worst unit. That’s on us. Our bad.

The Patriots’ defense could and should still rebound, but after four weeks they’re dead last in total defense and 31st in points allowed behind only the Indianapolis Colts, whose unit fittingly features five Patriots castoffs. It’s basically impossible for the Patriots’ defense to top the league in points allowed — as they did just last season — by the end of the year after such a horrid start.

So, what happened? Why and how did we overrate and overhype the Patriots’ defense so badly?

It starts with that stat we just cited. The Patriots ranked first in the NFL in points allowed last season. They also were good enough to win Super Bowl LI. The Patriots’ defense was slightly overrated by their lack of strength of schedule, however. The Patriots’ defense actually was average last season, according to FootballOutsiders.com’s DVOA metric.

On paper, it seemed like the Patriots’ defense should have been at least as good in 2017, if not better. They seemingly upgraded by replacing cornerback Logan Ryan with Stephon Gilmore, re-signed linebacker Dont’a Hightower, defensive tackle Alan Branch and free safety Duron Harmon and replaced Jabaal Sheard and Chris Long with Kony Ealy and two draft picks. The Patriots also boosted their interior depth with Lawrence Guy and linebacker corps with David Harris. Add in some implied improvement from defensive tackle Vincent Valentine, linebacker Elandon Roberts and cornerbacks Cyrus Jones and Jonathan Jones, all rookies in 2016, and the Patriots’ defense appeared to be shipshape.

Then edge defender Rob Ninkovich retired, depleting the Patriots’ front-seven depth. Rookie pass rusher Derek Rivers, Cyrus Jones and Valentine were placed on injured reserve, and Ealy was released because he never quite fit the Patriots’ defensive system. Hightower missed all of training camp and preseason then reinjured himself in Week 1. Branch looks like a shell of the player he was in 2016, and he’s barely playing because of it. Harris also has played just seven snaps and was inactive in Week 4. Throw in Gilmore’s serious communication issues in the Patriots’ defensive backfield, and that’s how the NFL’s best defense turns into its worst.

It’s possible we, and the Patriots, underrated continuity in the secondary. The Patriots experienced few, if any, communication issues when Ryan was on the field. Now Patriots defensive backs are tripping over themselves any time a group of receivers is in a bunch formation.

The Patriots are being forced to start undrafted rookie defensive lineman Adam Butler, who was a summer stud but has regressed since seeing his usage expand. Fourth-round draft pick Deatrich Wise has been a nice surprise, but that he’s playing as much as he is just shows the Patriots’ lack of edge depth.

The Patriots were forced to trade for cornerback Johnson Bademosi and defensive end Cassius Marsh before roster cutdowns to ensure even more undrafted rookies weren’t part of the Patriots’ defensive rotation.

Now, to play the Peter Gammons game, if Hightower can stay healthy, and if Gilmore’s communication can improve, and if Branch can start playing like he did in 2016, and if Wise can continue racking up sacks, and if either Eric Rowe or Jonathan Jones can emerge as a dependable third cornerback, and if the Patriots can add another piece before the Halloween trade deadline, then the Patriots’ defense still has the talent to be above average.

That’s a whole lot of ifs, though.

Thumbnail photo via Greg M. Cooper/USA TODAY Sports Images