What’s up with Marcus Smart’s defense?

Lower-body injuries are no joke.

For Marcus Smart, he spent much of this winter dealing with a calf strain, which held him out of the lineup for eighteen games. His return in early-March was welcomed, and he’s been playing heavy minutes as the key defensive anchor of this Boston Celtics team.

As goes Smart, so goes the Celtics’ defense. In the eighteen games Smart missed, the Celtics gave up 112.4 points and opponents committed 12.5 turnovers per game. Before his injury, their defense was much stingier: 109.2 points per game, and 14.9 turnovers. The drop in their effectiveness on that end largely coincided with the absence of their defensive leader.

But since his return, the numbers haven’t returned to early-season levels. Smart’s individual metrics remain solid, though there are several instances where Smart looks a half-step slower at the point of attack. The question is this: will Smart’s return to the form stabilize the leaking ship of the Celtics defense, or is there something still hampering him from operating at his peak?

All this comes from a place of confusion. Smart is one of the most consistent, mentally dialed-in competitors in the league on a nightly basis. He’s strong and stout, always willing to bang with guys on the interior. It’s been a huge asset to the team in the past, boasting a pseudo-point guard on offense who isn’t afraid to defend anyone in the post. As Smart ages, the league is starting to trend smaller, with more speed and athleticism than size. Opponents don’t look to beat the Celtics’ switching with back-to-the-basket mismatches anymore. They spread the floor, pound the ball and look to attack off the dribble.

That means less wrestling and more ballet for Smart, who switches onto speedy point guards more frequently than not. What we aren’t used to seeing from Smart, which has propped up on a few occasions of late, is him getting blitzed off the bounce by someone of his own athletic caliber:

Guys like Josh Hart or Keldon Johnson shouldn’t be taking Smart to the rack off the bounce. He’s developed a reputation as a brick wall, a forcefield that drivers bounce off of when they drive into his chest. This year, he’s struggled to stay in front of guys — it’s hard to drive someone into a brick wall when the road swerves around it.

Brad Stevens plays a switching style with his wings and guards, where Smart can exchange matchups with Jayson Tatum and Jaylen Brown. The switching neutralizes advantages through screens and forces isolations. It gives them a chance to proclaim inside position and dare guys to beat them individually.

The Celtics don’t really have a switchable 5-man on the roster, though. Tristan Thompson, Mortiz Wagner, Robert Williams, Luke Kornet... they’re all interior-bound bigs who Stevens would seek to prevent from a switch. When their man goes to set a ball screen, the Celtics play “ice” coverage, forcing the ball towards the closest sideline and funneling it towards their center, who is patiently waiting to bracket the ball and force a pass. On some middle ball screens, they’ll run something similar called “weak” where they push the ball handler towards his weak hand and cover the screen in the same fashion.

This is where some of Smart’s few lapses have been of late. The coverage is firmly dependent on the man guarding the ball forcing it away from the screen. Once the ball gets to cross sides of the floor, the defense is dead: it’s not built to recover well from this failure.

Last year’s first-team All-Defense representative from these Celtics shouldn’t be failing to keep the ball towards one side in ice coverages:

There could be a reason for these breakdowns that we, the common viewer, can’t ascertain: communication. Smart’s ability to deny the ball from going to the middle is dependent on him hearing the call for the screen come from Thompson. If the call isn’t loud, early, or continuous, Smart may be unaware of the coverage, then late to react to it.

The ball screen defense isn’t designed to have guards necessarily get through screens quickly and shut down plays, though. The expectation here wouldn’t be for Smart to smother the ball and prevent the handler from driving. It’s more of a bait-and-trap maneuver, where they encourage opposing handlers to speed up and attack the big. Once they commit to that, they’ll either run out of space and leave their feet (to no avail) or chuck up a heavily-contested floater in the lane.

Thompson and Williams do a really good job establishing verticality. When Smart does his job and forces the ball where they want it to go, good things tend to happen:

There is a downside to playing this type of ball screen coverage: the offensive glass. Smart’s job is to try and recover to his man to prevent the switch and contest a shot from behind. As his man speeds down the lane and draws the attention of a rim-protecting big, Smart is caught in no-man’s land, where he isn’t impactful at either contesting the shot or blocking out the big.

The result: second-chance points or silly fouls on Smart:

The C’s are willing to make the gamble that this style has rewards that outweigh the risks. By forcing the ball to the mid-range, they are betting that a turnover or missed shot happens more often than a make or a putback. And so far, they’re right, thanks to a pretty generous turnover margin.

Of all PNR on-ball defenders who have guarded 100 ball screens or more, Smart is 15th in turnover rate, forcing a change of possession on 17.3% of the screens he faces. Tatum is first in the league at 25.2%, and Jaylen Brown is 33rd. Ironically, while the Celtics haven’t been a fantastic defensive unit all year, they’re 7th in the league in turnover rate when guarding ball screens. They’re still above-average here, and Smart is a big piece of that success, even with a couple of miscues.

Other than a few uncharacteristic slip-ups when guarding the ball in space, Smart isn’t a culprit of Boston’s struggles. He’s also not designed to be in a position to be their savior either. As he continues to find his legs and return to 100 percent after a finicky calf injury, there will be some moments where he doesn’t appear as athletic as he’s always been, particularly laterally.

Smart is fine and, all things considered, is the least of our worries in Boston.