How good can the Celtics defense be?

With all the turnover on the Celtics roster, we’re trying our best to project the level of competence for the team on both ends of the court. If you missed it, you can check out a deep dive into Boston’s offensive potential here. Today is all about defense, a particularly more complex topic. Where the impacts of the team's new roster seem fairly obviously positive on offense, the same is not true defensively.

Let's start with reasons for concern. The Celtics lost three of their best defenders in Avery Bradley, Jae Crowder, and Amir Johnson. That's one of the best one-on-one perimeter defenders in the world, an extremely switchable, rugged wing, and (statistically at least) the team's best deterrent at the rim. Those are some pretty major losses.

The Celtics’ sea change of personnel will allow for some interesting structural changes (more on that momentarily), but at the end of the day, Boston has a little less individual defensive talent on the roster than they did one year ago. That will always matter, and it could hurt them in the aggregate.

Rebounding will be a challenge too. The Celtics’ troubles on the glass are a bit overstated at times, but an Al Horford/Marcus Morris front court holds the potential to be not just the worst rebounding duo in the league but in the history of time (based off of last year’s statistics anyway).

They’ll also need to find a way to effectively protect the rim. Horford is the presumed front runner to fill that role. He’s big and athletic and is a relatively capable shot-blocker, but opponents only shot 2.5 percentage points worse on field goal attempts defended by Horford within 6 feet of the hoop. For context, Amir Johnson led the team with a differential of negative 10.6.

Those numbers, while meaningful, don’t tell the whole story. When playing beside Johnson or Kelly Olynyk, Horford was frequently tasked with chasing around opponents’ stretchier bigs. His instincts to protect the basket didn’t disappear in those instances, but his positioning out by the three-point line semi-frequently inhibited his ability to act on them.

It’s hard to say just how often plays like the above truly effected Horford’s ability to keep opponents from scoring. His overall numbers on shots defended in the paint mirrored what he’s posted for the past several years. That may just be the player he is now, and that’s OK. Defense is as much about keeping opponents from getting to the basket as it is about turning them away once they get there.

Horford was the anchor of some incredible defenses in Atlanta, and he didn’t fare much differently as a shot-alterer in his final seasons there. He should be able to play that role more robustly, given Boston’s inclination to start Morris beside him. Morris can handle any opposing small-ball 4s and leave Horford to patrol the paint. If he can help organize the Celtics’ back line and challenge shots by the basket a little more effectively, Boston’s defense should at least be decent.

There are some other reasons for hope as well. The Celtics’ worst defender, Isaiah Thomas, is no longer in the fold, and while his replacement, Kyrie Irving, does little to inspire confidence on that end of the court, he is tall enough to make switching pick-and-rolls a more viable defensive strategy. That's something that Boston is likely to embrace. They've built a roster full of long, athletic defenders that should be able to swap defensive assignments with relatively limited consequences.

Think something a little like this:

It's difficult to break down a defense that doesn't need to work through picks (on and off ball). With its roster constructed as it is, Boston can predicate its defense on creating a switchable shell around the perimeter and disallowing penetration. That's more or less what they've tried to do in recent years. The difference will simply be in strategy. Whereas the Celtics relied on the individual brilliance of Avery Bradley and Jae Crowder in years past, they will likely lean more on schemes and collective versatility in the coming season.

It's hard to project exactly how that might play out. Switching is a great tool to have in one’s belt, but if that’s the only option Boston has at its disposal, they could be in trouble. Swapping defensive assignments leaves teams vulnerable to mismatches to be exploited in isolation, and having a single, set defensive approach makes game planning to beat it a much easier task. The Celtics will need to at least try to work through screens without switching, if for no other reason than to keep opponents on their toes.

There is a world where Boston improves its defensive efficiency from last year, but it is contingent upon growth from Jaylen Brown, a major uptick in effort from Kyrie Irving, and Al Horford’s ability to make would-be drivers think twice. If those variables break the wrong way, the Celtics could be bad defensively.

There's probably a little too much defensive talent remaining for that to happen. Ending the year somewhere around average to slightly above average seems more probable. That's about what they were last year, and setting a similar expectation for the upcoming season, with a slight discount for the transactions of the summer, is reasonable.