BOSTON — The drama arrived late, but with enough intensity to deliver on the game’s immense promise.

The matchup between the defending champion Boston Celtics and the undefeated Cleveland Cavaliers lived up to expectations as one of the most important games of this young regular season. Boston dominated the first half. Cleveland, looking to keep its perfect season intact, stormed back with a big run in the third quarter. As if to announce to everyone inside TD Garden that his team would not go down easily, if at all, Donovan Mitchell clapped his hands vigorously after taking the Boston lead early in the fourth quarter.

And then the Celtics, after losing their way early in the second half, had to figure out how to survive the confident, surging Cavaliers. With the action on the field chippy and Cleveland’s defense not allowing ground, buckets were not easy to produce. To salvage a 120-117 victory, Boston needed Jaylen Brown and Jayson Tatum to overcome all the Cavaliers’ defensive aggression.

“I thought they both handled the pressure and the physicality well,” Joe Mazzulla said. “And we quieted the offense. We played a little slower, but it was more important to get the right distance. It was more important to get our drive-and-kick reads. And they were able to find a good balance between handling the pressure, driving to attack, driving to get to the free throw line, driving to find guys. So I hope the teams stay physical because it just gets us reps, and I thought they handled it really well.”

The Celtics’ skill often carries them. They have made more three-pointers in the first fifteen games (287) than any team has ever lost in a fifteen-game span. But to create those opportunities, a team must pressure the rim. In Tatum and Brown, Boston has two of the biggest and strongest wings in the league. When they play to their strength, as they did for much of Tuesday night’s game, an opponent better boast size across the board.

It wasn’t all the Cavaliers’ fault for not having enough. With Dean Wade, Isaac Okoro and Caris LeVert all injured, the Cavaliers were missing most of their players with the combination of length, muscle and mobility to slow down the Celtics’ two All-Stars. Early on, it became clear that Cleveland would be in trouble against Boston’s penetration. Even solid individual Cavaliers defense wasn’t enough at times.

Four minutes into the game, Tatum tried to drive past Sam Merrill once, but was unsuccessful. Tatum started to try again, but Merrill stayed ahead of him a second time. Given Tatum’s size advantage in the game, Merrill, who wasn’t necessarily known for his defensive impact, did an admirable job staying in front of the All-Star and keeping him away from the rim.

It didn’t matter. Just by being such a big threat, Tatum had already created the advantage his team needed. Based on the defensive alignment of the other Cavaliers, they were deeply concerned about Tatum’s chances of penetrating the paint. Their fear of what he might do to Merrill allowed Brown to open one death. Tatum didn’t have to actually beat Merrill to beat him. After Mitchell crawled off Brown to shield the paint, Tatum created an open 3-pointer with a simple pass to Brown.

The Celtics targeted the Cavs’ smaller guards. Darius Garland was Jaylen Brown’s main defender. (Winslow Townson/Getty Images)

At their best, the Celtics viciously expose a defense’s vulnerabilities. On Tuesday, they often targeted Cleveland’s smaller guards. Three of Brown’s eight assists came with Darius Garland as his primary defender. After Cleveland got within two minutes of the fourth quarter, Boston chased that switch twice. Brown drove slowly to a spot about 10 feet away from the basket before bunting Garland twice to create enough space to shoot down a short jumper. Although that attempt failed, the Cavaliers were unable to secure the offensive rebound. Brown later took advantage of the same game in possession. He drove left, beat Garland, drew the defensive help and found Tatum in the corner. Brown’s way of forcing Cleveland into scramble mode allowed Tatum to beat a Jarrett Allen closeout and finish a dunk that put Boston ahead by four points.

Tatum and Brown scored all 12 of their combined points in the fourth quarter at the rim and free-throw line. They combined for five of Boston’s six assists in the quarter; three of their assists came on paint touches by them. When they went to the rim with power, they created a big offense for their team.

While Brown and Tatum still settle for difficult outside jumpers at times (think a pair of Tatum’s 3-pointers in the second half), they’ve also taken their physicality to the next level this season. Maybe it’s because they had to do it; Mazzulla believes the league as a whole is playing with more vigor. Maybe it’s because they are older, stronger and wiser now. Maybe it’s because teams, looking for a way to disrupt Boston’s high-powered offense, do their best to bump them, push them, and challenge them. That could have consequences for Brown and Tatum if they allow it.

“They’re trying to speed us up and create revenue,” Brown said. “Because if you don’t do anything, just sit back and let us take you apart. That doesn’t work. So it makes sense that they try to be physical, speed up the game a little bit. But we just have to deal with it, get the coverage right, take our time, be patient and trust the game and our teammates.”

For the first time in Tatum’s seven-year career, the Celtics are on pace to finish in the top half of the free throw leaderboard. Fourteenth place may not seem like much to brag about, but after years of being near the bottom of the rankings in that category, they are finally adding free throws to their long list of ways to score. The biggest difference comes from Tatum (8.5 free throw attempts per game) and Brown (7.6), who are on pace to reach new career highs in free throw attempts per game. Brown is on pace to break his previous career best (5.1). After building muscle this offseason, he entered Tuesday ranked second in the NBA in points per possession on post-up attempts, thanks in part to an outsized number of free throws on such attempts. He has leaned into the bully ball and overwhelmed smaller defenders.

Brown has also used his strength to set up teammates for quality shots. Three minutes into the fourth quarter, with the Celtics holding a three-point lead, they made a play to get it in the post against Garland. Instead of forcing his own offense, as he might have done early in his career, Brown calmly worked his way through the paint. That forced Allen to move into the shadows on defense, freeing Neemias Queta on the baseline. Queta drew a foul after Brown dumped it on him.

Brown set up his teammate to score by playing through the physicality of the defense.

“Just managing it, taking care of the basketball and still finding the right reads,” Brown said. “That’s it. Just be patient. Be really patient.”

Fair or not, the scouting report on the Celtics always said they would unravel in the game’s biggest moments. That they wouldn’t trust their team’s offense when it mattered most. That an opponent could confuse them by turning up the intensity high enough. They have solved many of their old problems and continue to evolve.

They didn’t like the way they played in the third quarter as they let most of the 21-point lead disappear, but Mazzulla said they still accomplished his top priority.

Jayson Tatum fouls Donovan Mitchell in the fourth quarter. (Winslow Townson/Getty Images)

“We were able to tighten up on some things in the fourth quarter that we gave up in the third quarter, and we just showed a level of physicality that we can achieve on both ends of the floor,” Mazzulla said. “I thought the boys did a great job. I thought we were physical the whole game, what I care about more during the regular season is our physicality, our toughness. The details are the second most important thing, and we missed a little bit of that in the third. But in the fourth we picked up both things. But I thought we were extremely physical defensively, and I thought we were very balanced and intentional offensively. And I thought we had written good books.”

Even as he handed the Cavaliers their first loss, Brown said the Celtics “could have played a lot better.” He believed they took their foot off the gas after dominating the first half.

They could still manage the game with their physicality. Ending Cleveland’s 15-game winning streak must have felt nice, but Al Horford said the Celtics’ focus was on themselves.

“It’s all about us,” Horford said. “It’s not about anything (else). It’s not about proving to the league or anyone else. It’s a long season and we continue to make progress to get better.”

(Top photo of Jayson Tatum going to the basket: Winslow Townson/Getty Images)