Basketball is a game of runs. That doesn’t mean you sit back and change nothing, just waiting for the next run to happen.
The Boston Celtics seemingly were in control in the middle of the second quarter on Wednesday, leading 41-32. Then the San Antonio Spurs went on a melee, outscoring Boston 29-6 to close the quarter. They made their last ten shots - which has some luck involved. What was more notable was how they were out-muscling the Celtics, putting their shoulder into their chests and going at them one-on-one.
The lack of physicality from the Celtics carried over after the break. Less than five minutes into the third, the lead ballooned to fifteen after a LaMarcus Aldridge offensive rebound and putback. The Celtics were giving up far too many layups or dunks, so Brad Stevens had to do something.
His grand plan: switch to a zone defense. While Brian Scalabrine called it out as a 2-1-2 zone on the broadcast, it’s the NBA’s modern solution to a 2-3 zone while dealing with the defensive three seconds violation. Whatever its formal name, the zone was effective.
Instantly, as the Celtics were less concerned with letting their man drive past them and more about funneling the drive to their teammate, the Spurs became less aggressive. The Spurs had only one shot at the rim for the final seven minutes of the quarter - a missed putback attempt by Keldon Johnson.
The result of this change: the Celtics retaking the lead by the end of the third quarter.
The zone’s purpose is to influence the ball to be driven to one of two areas: directly to Tristan Thompson stationed at the free throw line, or towards the baseline to switch responsibilities from the top of the zone to the bottom.
When the C’s made their switch, you could see the Spurs sputtering to figure out how to attack. Their first possession featured a rim attack that went right at Thompson, the Celtics drop around him and the Spurs were forced to a kickout jumper:
On a night where Thompson was the most reliable defender to meet drivers with his chest and challenge a shot, the Celtics switched to a scheme that funneled handlers directly to him.
For any zone defense to work effectively, it takes communication to match to cutters and prepare for overload. The rules of a zone at this level are far less concrete than lower levels of basketball. That’s particularly applicable to the bottom three on the baseline.
Sometimes they have to pass off cutters and communicate. Sometimes they have to move with the cutter and abandon their area. The cardinal sin is in guarding nobody.
Watch this possession, where Thompson does the same thing as the one earlier: steps up to accept the driver attacking middle off a ball screen. Jaylen Brown ends up being the savior, sliding with a cutting Rudy Gay because he realizes nobody else can mark Gay:
These fluid rules mean that the zone acts like an amoeba based on what the offense does: it changes shape. Thompson’s responsibilities are first and foremost to not leave the rim if there’s an offensive player there; the high post and free throw line is secondary.
When Thompson is guarding in the lane, the zone looks more like a 4-around-1 matchup zone, as the Spurs lacked the movement through the lane to create overloads. That made it pretty easy for the Celtics to stay with their assignments, limit their movement and force tough mid-range jumpers.
Stevens knows going to a zone is a bit of a cop-out for his team though. He was not pleased with his team’s man-to-man effort and bailed them out with a strategic switch. But he knows that long-term, the Celtics need to solve their man-to-man defense to be a true contender. As he noted in his postgame presser, “I’d like to be able to play multiple defenses well but to do that, we need to play at least one defense well.”
The zone was a nice stopgap, but it’s not the solution to their problems, and ultimately was not enough to win the game on Wednesday night.