The success of the 2018 Chicago Bears was driven by a defense that was at the top of the NFL. Exactly where they ranked depends on which metric you prefer, but Football Outsiders ranked them first overall, first against the pass, and second against the rush. That seems about right. In a perfect world, the Bears would ride the best defense in the NFL into the history books, remaining at the top of the NFC North for years. This would allow Matt Nagy the chance to develop Mitchell Trubisky and the offense, and all would be well.
It’s probably not going to happen. The defense is likely to see a dip in performance, and that’s even assuming that they don’t lose any important players like Adrian Amos or Bryce Callahan. When that happens, some fans are likely to blame the loss of Vic Fangio and the introduction of Chuck Pagano. The thing is, it won’t be Pagano’s fault, necessarily, either.
Defenses are really inconsistent on a year-to-year basis. This is a major reason it makes more sense for a team to invest in offense, and it’s also one of the reasons why the Bears are fortunate in having an offensive coach at the center of their organization.
Turnovers Aren’t Sustainable
Football Outsiders tells us that predicting turnovers is a difficult thing. In fact, it’s really unlikely for a team to sustain turnovers on a year-to-year basis. In 2004, it was noted that teams rarely sustain a presence at the top of the turnover board from one season to the next, and that exact same trend was found to be true again in 2014.
For 2018, Chicago led the league with 36 takeaways. That is really, really good. It also does very little to suggest how the next season is going to go.
To pick one finding out of many, 538 reports that from 2009-2018, a team’s interception rate in the prior season had only a 2.4% share of predicting the next season’s success. What Kyle Fuller did in 2018 was great, but there is little reason to think the Bears are going to stay at the top of the leaderboard in this regard.
What this means is that Chicago’s defense--which had five more takeaways than the next-closest competitor in 2018--is unlikely to stay that dominant. This won’t be Pagano’s fault, even if fans look to find a way to blame him.
Chicago Over-performed in Sacks
Unlike interceptions, quarterback hits do tend to be consistent on a year-to-year basis. Sacks, however, do not (and sacks only own a 3.6% share of predicting the next season’s success rate). Chicago was 8th in quarterback hits last season, with 95; that should have resulted in an expected sack rate of around 41 or 42 sacks. Instead, the Bears enjoyed 49 sacks.
In other words, while quarterback hits are consistent, sacks are not. We should expect to see the same defense get slightly fewer sacks next season even if they do maintain the same rate of hitting the quarterback. That means that Chicago will likely field a top-half or even top-third defense. It does not suggest that they will be one of the very top teams in sacks, like they were in 2018.
Defense as a Whole is Volatile
In general, defense is less reliable than offense. That’s one reason why offensive DVOA does roughly twice as good of a job of predicting the next season’s success as defensive DVOA. The top three defenses in 2016 were Denver, New York (Giants), and Arizona. In 2017, those same units ranked 10th, 18th, and 4th. In 2018, there were--in the same order--5th, 24th, and 17th.
A number of factors impact defense, including rules that are slanted to favor offenses and the ability of offenses to isolate aspects of a defense that they choose to focus on. In simple terms, an offense can almost always choose who gets to be in on a play, even if that choice is filtered through the opportunities provided by a defense. On the other hand, a defense cannot be guaranteed that its best players will even have a chance to make a play on the ball.
Ultimately, what this means is that when the defense of the 2019 Chicago Bears take the field, they will likely be a capable unit. They will probably be able to take over games and win a few contests almost on their own. However, it is really unlikely they will dominate games as they did in 2018, and their struggles will have a number of explanations unconnected to either Vic Fangio’s departure or Chuck Pagano’s arrival as his replacement.