There's been a lot of handwringing on Celticsblog about the sorry state of the team's defense so far this season. And rightly so. After finishing in the top five in defensive efficiency in each year of the Garnett era (and usually first or second), the Celtics are now giving up the 21st most points per possession. That huge drop, if sustained for the whole season, effectively takes the Celtics out of contention for the title. Even with a much-improved offense, that sort of defense effectiveness wouldn't allow the team to beat the juggernaut offenses in Miami, New York, and Oklahoma City, among other places, four times in seven games.
So what's the problem with the defense? Well, there are several factors that could explain the dropoff, including a decrease in the number of forced turnovers. But after going through some data, I think the defensive decline is mostly attributable to two factors: opponents are taking and making a lot of shots both at the rim and from behind the three-point line. Of course the smart people at this blog have noted the former many times, but they've talked less about the latter. Both, I think, are things the team needs to fix if they want to be players in May and June.
The Celtics defensive philosophy over the last six years has been to zealously guard the paint and to harass three-point shooters. It's a philosophy as smart as it is simple: make opposing teams take lower-percentage, lower-reward long two-point shots. And they have been remarkably effective at it: in the previous five seasons, the Celtics were top five in the league in the number of shots opponents took at the rim and the percentage of them that they made; the same goes for three-pointers.
This year? Not so much. So far this season, the team has given up the 12th-highest number of shots at the rim; incredibly, they have the fourth-worst percentage allowed, at 67.5%, a mindboggling change of character for this squad. This team went from being fanatical rim protectors to being pillowy-soft in a Cleveland-Charlotte kind of way.
Several people on this blog have posited that the dropoff in interior defense is due not to a decline in the skills of our core players but in teaching the new players the rotations and effort that are required to protect the rim at the level the Celtics demand. The stats would seem to confirm this view: Celtic opponents have the second-highest number of at-the-rim baskets that have come from assists, at 57%. This would seem to indicate that the problem isn't that the team is getting bullied in the post or getting beaten one-on-one but that other teams are very successful at breaking down the defense in the paint with smart passing, and thus getting easy baskets.
In other words, what we have all noticed is borne out by the stats: the Celtics' rotations and help defense have been slow, inadequate, or just wrong all season, resulting in too many easy points for our opponents. Fortunately, given the high assist rate, this seems fixable, less a problem of inherent weakness and more a problem of practice and familiarity.
The three-point defense is similarly miserable. The team is actually not allowing a lot of three-pointers, relative to the league average, but the eFG% of those shots is painfully high at 56%, sixth-worst in the league. Again, this is unprecedented, as the Celtics have been in the top five in opponent three-point percentage over the last five years.
Why the change? Well, logic and watching the games would indicate that the slow and wrong rotations have been a problem here as well. The Spurs and Heat games would be exhibits A and B in this argument. But I think there might be an additional explanation: bad luck. So far this season, Celtic opponents are assisting on just 76% of their three-pointers, well below the league average of 81% and even further below other woeful three-point-defending teams. This low percentage suggests that opponents are not making a lot of easy, open threes but that the shooters are creating their own shots and making a very high percentage of them. In other words, despite some noticable lapses, in general the Celtics are doing a good job of defending three-point shots. The problem is that other teams are just making a high number of usually low-percentage (i.e., shooter-created) threes.
Hopefully that bad luck will correct itself over the course of the season. Hopefully.