Late in the third quarter of Game 1 of the Eastern Conference Semifinals, Marcus Morris took one dribble from the top of the key and rose for a pull-up jumper at the right elbow. In the paint, Marcus Smart boxed out J.J. Redick, potentially primed to snag an offensive rebound. Behind those two guards, Joel Embiid lurked.

As the ball caromed off the back rim, with Redick neutralized, Smart and Embiid both went up for the board. Smart, despite standing 10 inches shorter, snatched the ball from Embiid’s clutches and came down with the rebound. Then, he contorted his body, elevated, absorbed the contact and converted the putback over the 7-foot-2 unicorn. The sequence lit a fire inside TD Garden, producing a sort of intangible energy:

In the first four games of the 2018 playoffs, the Celtics’ vaunted defense craved that intangible energy Smart provides on both ends of the floor. Against the Bucks, Boston had sprung myriad leaks with a 113.9 defensive rating, the second-worst mark of any playoff team and a far cry from their league-leading clip of 101.5 from the regular season. Since Smart’s return, Boston has registered a 99.9 defensive rating, trailing only the Warriors over that span. Individually, Smart boasts a 102.2 defensive rating this postseason and his on-off split defensively is the best on the team. Without Smart, the Celtics’ brigade has ceded 109.0 points per-100 possessions.

With 5:38 to go in the second quarter of Game 5 of the Eastern Conference Quarterfinals, Jabari Parker took a handoff from Tyler Zeller and curled around a screen as the crowd grew deafening — though it wasn’t because of a devotion to Parker.

“Marcus Smart about to check in,” said Gus Johnson, Bucks play-by-play announcer.

The 6-foot-4 bulldozer in sneakers stood postured on the sidelines, tied the drawstrings on his shorts and adjusted his royal green waistband. After a six-week absence, Smart’s return to the court was imminent, awaiting the next dead ball.

Smart’s defensive malleability has unlocked some matchups previously unavailable for Brad Stevens and Co. He’s chased elusive and shifty marksmen such as Khris Middleton, J.J. Redick and Marco Belinelli around the arc while also battling against purebreds Embiid and Giannis Antetokounmpo in the post.

Against the Bucks, Smart regularly took on the challenge of guarding the hooper-turned-flamethrower Middleton. The 6-foot-8 lanky wing was on another stratosphere as a scorer in round 1 and still poured in 23.5 points on a .574/.591/.917 slash line in the final three games of the series. Smart, though, certainly made things less comfortable for him.

Watch here as he ICEs the pick and roll against Middleton and then doesn’t allow him to turn baseline for the fadeaway — one of his preferred shots over shorter defenders — and forces an awkward jumper at the elbow where help is looming:

This was not an isolated case, either. In both rounds, Smart has routinely disrupted possessions, sniffing out the intended action on a given set:

Smart also quarterbacks the Celtics’ stingy defense. He coordinates switches, identifies assignments in transition that need to be picked up and seamlessly bounces from man to man without a hint of confusion in half-court sets. His ability to direct Boston’s younglings in transition situations has stymied more than a few fast break chances for opponents in the playoffs, forcing them to resort to a half-court offense.

Where Smart’s return to the court has perhaps most benefitted Boston defensively is as an off-ball defender, highlighted, though not confined to, his nine combined steals and blocks through four games. There aren’t many traditionally sized guards who could provide rim protection, but Smart has this postseason, exhibiting a keen sense of timing on rotations:

It shouldn’t be possible for the 6-foot-4 Smart to break up a lob to a 6-foot-11 Antetokounmpo sporting go-go gadget arms and force a turnover, but much of Smart’s game seemingly defies logic.

His ability to buck conventional wisdom carries down to the block, where he’s stonewalled the likes of Antetokounpo, Embiid, Middleton, Parker and Ben Simmons. With quick hands, a tank-like frame and acute defensive guile, Smart’s ensured that his height hasn’t been a liability down low.

Note how, with sheer lower body strength, he moves Parker off of the block, forcing him to catch the ball in a middle ground between the paint and the three-point line. Then, when Parker lowers his shoulder to get downhill and rumble to the rim, Smart doesn’t budge, which induces an ill-advised midrange jumper from the baseline.

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Defensively, Smart’s impact has manifested across the stat sheet this postseason. His on-ball defense has made opponents more uncomfortable than a middle schooler on their first date.

Watch as his pestering defense again forces Middleton to ignore the screen. As Middleton takes his eyes off Smart and attempts to navigate around the Thon Maker pick, Smart swipes the ball free to spark a transition bucket, lobbing it up to Al Horford for two:

For all the praise Smart’s defensive acumen deservedly gets, his playmaking chops have been almost equally necessary for Boston so far.

Horford might be the league’s best facilitating center this side of Nikola Jokic, but Kyrie Irving’s absence has forced him to become more of a scorer in the playoffs. Jayson Tatum has flashed some playmaking brilliance throughout his rookie campaign, but can’t be relied upon to commandeer the offense on a regular basis while Rozier is best suited to inflict damage with his scorer’s mentality — though he’s been stellar thus far with a tidy 6.6:1.6 assist-to-turnover ratio.

That’s where Smart, averaging five assists per game this postseason, enters the equation. Sliding Rozier and Tatum off the ball allows those two to spot up on the perimeter and slash to the cup as Smart handles playmaking responsibilities. It also alleviated the burden Rozier was tasked with as the only consistent creator in the backcourt during the first four games against Milwaukee.

Smart has the vision and gusto to make reads that his teammate can’t necessarily always recreate.

Watch as Boston runs a double-drag screen for Smart with Rozier as the first screener:

He navigates around both picks and just as the Buck defenders turn their attention toward the rolling Aron Baynes and two shooters on the right side of the floor, Smart slings it back to Rozier on the left wing for a wide-open three-pointer.

Another time, the Celtics ran a simple pick-and-pop between Smart and Horford:

Eric Bledsoe cheats off Rozier to defend against a potential three from Horford — or maybe it’s because he doesn’t know who Rozier is (hah, jokes). But anyhow, Smart freezes Bledsoe with some misdirection before firing a pass to Rozier, who has relocated from the wing to the corner.

Between the two point guards, Rozier is better equipped to play off the ball as Smart is still recovering from the injury to his thumb and has never been a plus shooter. Without another competent lead guard in the rotation, it was hard for Rozier to slide off the ball, aside from when Horford operated as a primary facilitator.

Below, the Celtics crowd the right half of the court, affording Tatum an opening in the paint after he curls around Rozier’s pindown screen. However, the play isn’t possible without Smart’s dime as he rifles it to a cutting Tatum in stride for the layup:

From March 14 to April 23, the Celtics were without one of their top all-around defenders and secondary playmakers. When he hopped up from the bench and made his way to the scorer’s table midway through the first quarter in Game 5 against Milwaukee, fans were right to stand and cheer. His return has rekindled Boston’s top-notch defense while also adding some offensive pop to the team’s backcourt rotation, even without Irving.

For all of Brad Stevens’ excellence, Smart’s game isn’t one that the next man off the bench can recreate. So, if the feisty, chippy, unique guard was to be coronated by applause each and every time he enters the game this postseason, it would be warranted.