How bad are they?
The San Diego Chargers have the second best offense in football this year, a special teams unit that is almost exactly average, and they've faced one of the easiest schedules in the NFL. Despite those facts, they currently sit at 5-6 after 11 weeks of football. We've addressed how Mike McCoy's game management has hindered their chances, but not going for it on 4th downs isn't why this team has a losing record.
The defense of the Chargers is downright horrible. By DVOA (a stat that takes into account strength of opposition, down, distance, score, etc.) they're the fifth worst defense through 11 games in the history of the stat which goes all the way back to 1989. They're bad everywhere as they're dead last in DVOA against both the run and the pass. Their lead on the 31st ranked defense against the run is particularly impressive.
Maybe you're not a fan of advanced stats because you don't trust proprietary metrics (or maybe you just don't understand math; no judging). In that case, let's take a look at the unadjusted stats from nfl.com. The defense allows the fourth most yards per game. This actually understates how bad they've been because the offense is able to win the time of possession battle for them. They actually allow the most yards per play. They've forced the fourth fewest fumbles and intercepted the fourth fewest passes. And they've allowed the second highest QB Rating.
Luckily for the defense, the offense and special teams have gifted them long fields and limited the amount of possessions they face. This has helped keep them middle of the pack in scoring defense and should be a helpful reminder of what a poor metric that is for measuring the competency of a defense.
Why are they so bad?
This is obviously more difficult (and more open to interpretation) to answer than how bad, but I'll try to address it somewhat. The most obvious issue has to be health. At present, the Chargers are missing their top four outside linebackers in Dwight Freeney, Jarret Johnson, Melvin Ingram and Larry English. That means that they're starting third stringers at both outside linebacker spots and the guys giving them a rest are fourth or fifth stringers (some of whom are playing out of position). Thomas Keiser and Tourek Williams are arguably pretty good for third stringers, but neither is good enough to start on an NFL team. Both are starting for the Chargers.
The defensive line is made up of guys that are bad fits for the defense John Pagano runs. Corey Liuget, Cam Thomas and Kendall Reyes all struggle when asked to defend two gaps. They are asked to do this on a significant number of snaps. All three are talented players, but few players can succeed when being asked to play to their weaknesses. Sean Lissemore and Lawrence Guy have both done an admirable job as backups, but they've only been in for about 180 snaps between them.
The inside linebackers both missed time due to injury. Donald Butler, in a contract year, has probably lost himself quite a bit of money. He has shown himself to be unable to stay healthy, and when he's been on the field, his play has not been that of an elite inside linebacker. At the Mike, Manti Te'o has simply been a disaster. He finally flashed signs of life against Kansas City, but even still had an absolutely lousy game. He struggles to read guards, read routes, and tackle. He can't get off a block to save his life and recently got trucked on the goal line by a running back he outweighed by nearly 50 pounds. The options behind him are arguably worse.
The corner play of this team has been dreadful. Johnny Patrick has arguably been the most productive of the bunch. None of them ever look back for the ball. They all show poor technique in nearly every facet of the game, and there have been zero signs of improvement. The safeties are an All Pro that has frequently been asked to play out of position, a corner that's trying to learn how to play safety and an undrafted free agent.
That all adds up to a really bad unit. Combined with the defensive coordinator's refusal (or perhaps an inability) to adjust to the strengths (such that they exist) of his personnel, we're getting to watch a historically bad defense. A team that features players as good as Eric Weddle, Liuget, Reyes and Butler probably shouldn't be historically bad, but at this point it's getting harder to make the argument that they aren't at least close to the least talented defense in the NFL. I wonder how the front office sees it. How do you see it?