In the zone: what makes Boston’s zone defense so great?

I’m not really a “sound the alarm” kind of guy. The Celtics haven’t played up to their ability level this season, on either end of the floor. Brad Stevens-coached teams always talk about their identity being forged on the defensive end, so struggles in that area are most concerning.

As of March 22nd, the Celtics have the 26th-ranked half-court defense, according to Synergy Sports Tech. Their man-to-man looks allow over one point per possession, give up a score on 45% of their possessions and have been lackluster in their energy and attention to detail.

When the C’s have been in big games and need to steal a few minutes on defense, they’ve gone to a stopgap measure, a morphing 2-3 zone. Boston went to it again on Sunday, spotting it for moments in time after giving up 31 points in the first 11 minutes against the Orlando Magic. It’s worked wonders for Boston, not only stopping the bleeding in the moment but consistently befuddling teams. 42 games into the season, that stingy zone is holding teams to 37.9% shooting, best mark among zones in the NBA.

As the Celtics still struggle to get stops, it begs the question: can this zone be more than a change-of-pace gimmick and actually a primary option against potent offenses?

Stevens is hardly the only coach to go to a zone. In fact, the Celtics are 8th in zone frequency, with only 4.1% of their defensive possessions lined up away from man-to-man. That’s far behind the league-leaders, Charlotte Hornets (14.1%) and Miami Heat (10.9%). What Stevens has embraced this year, that he hasn’t in the past, is using it within the flow of the game, not just in situations that would help thwart sideline inbound plays.

With so many teams around the league using a zone in some variety, the Celtics need to do something to make theirs a bit different. Something that keeps its potency.

With defensive three seconds rules, zones can often shapeshift during a possession. A 2-3 zone turns into a 2-1-2 when the middle man steps up to the free throw line to simultaneously cover a player there and reset the three second count. In lineups early this season with Tristan Thompson and Daniel Theis together, that was more frequent.

The Celtics have gotten away from that, dependent on personnel. When facing a stiffer big man who isn’t a great playmaking threat at the nail, the C’s will sag far off him, allow the ball to go to the high post and keep their rim protector stationed at the hoop.

It’s been really effective at putting Robert Williams where he can be most impactful, and daring teams to take the least efficient shot on the floor, leaving non-shooting bigs unguarded outside of eight feet:

Williams is the linchpin to any effective defense for Boston right now. His shot blocking prowess in the middle discourages rim attempts and encourages those welcomed floaters or 15-foot jumpers.

The Celtics construct their zone to keep Williams as close to the rim as possible. NBA offenses are smart, though, and know how to attack that type of coverage. They overload one side, put shooters in the corners and create 3-on-2 or 2-on-1 situations.

Stevens, thanks to the mobility of Timelord, is able to counter this. Whenever a ball screen occurs near the top of the zone, the low wing will sprint up to accept the ball. The Celtics anticipate the ball movement to the corner and send Williams to contest, knowing he has the athleticism and length to cover ground and contest shots when he arrives.

It’s a unique X-out that you only see in the NBA. The corner three is such a valuable shot that the only way to take it away is to send your middle man to it. The zone has been most effective lately with Williams as the anchor, knowing he can alter these shots and has a good feel for how to get there.

The first thing that makes the Celtics so effective here isn’t necessarily in Williams’ effectiveness, but in how the other guys around him help box out. Most teams have issues rebounding out of a zone, and there are possessions when Boston isn’t immune to those challenges. That said, rebounding is a lot easier when there’s an understanding of where the shot will come from.

As Timelord heads out to challenge those corner jumpers, it takes away one of Boston’s best rebounders from the rim. The others all chip in. The opposite wing slides to the middle to take away the biggest defender, and the top guy will chip down to finish the box or bury any crashers out of bounds.

It works with any lineup, but is particularly effective in two-big lineups, when Daniel Theis or Tristan Thompson are the ones sliding over to block out the center:

Another important facet of zone defenses is that they inherently feature man-to-man principles. A wise coach once told me that a good zone looks like man-to-man.

Enter Kemba Walker, an experienced veteran and terrific help defender. When Timelord gets beaten on penetration from those perimeter closeouts, the rest of the C’s are there to cover up. In clutch time against the Toronto Raptors, Walker slid over to take a charge in the zone, the same rotation he’d have in man-to-man, while Jeff Teague was ready to cover his assignment in the corner:

The zone, at the very least, makes opponents think. The rotations are a little funky with that Williams x-out to the corners. Teams haven’t gotten a great feel for what type of penetration can get Williams to step up and what will shrink the zone in ways the Celtics aren’t comfortable.

The result can be some pretty hesitant possessions where the offense doesn’t seem to be in control of what they want:

While the defense has been effective, it’s probably a little too gimmicky to be played full-time. It’s clearly best around Timelord in the middle and with wings who can help rebound. It can be a nice stopgap measure that allows Boston to play Theis at the 4 or steal some possessions when they need. More than anything, it’s an awesome trick up their sleeve for a playoff series that can be used strategically.