It’s official: There’s something wrong with the Celtics’ defense

Donovan Mitchell got what he wanted on offense, whether it was going downhill toward the rim or draining open jumpers.Barry Chin/Globe Staff

The Celtics’ lack of defensive intensity and execution were camouflaged by their season-opening success three wins. Monday’s whipping in Chicago was called an aberration.

After Friday, it’s safe to say defense is Joe Mazzulla’s first real issue as head coach.

If the Celtics make enough plays against the Cleveland Cavaliers, they win a high-scoring game. But they didn’t. The offense sputtered. Jayson Tatum stopped scoring. Jaylen Brown made turnovers. Marcus Smart missed threes.

It still would have resulted in a victory had the defense provided resistance to the Cavaliers duo of Donovan Mitchell and Caris LeVert.

They didn’t. Mitchell and LeVert took turns draining jumpers and driving for layups. On many occasions the Celtics missed assignments or went under screens. The 132-123 overtime loss was ghastly because it exposed what could be a fatal flaw in these championship aspiring Celtics: the inability to consistently defend.

They yielded 120 points last weekend in Orlando, another 120 in Chicago, and 114 in regulation Friday. More distressing was as the defense crumbled, the Celtics slipped on the boards. And Cleveland making shots prevented the Celtics run pushing the ball up the floor.

There were several breakdowns in the second half, after the Celtics raced to a 75-point first half. Mazzulla reiterated the Celtics could have won had they grabbed a couple of rebounds or taken care of the ball, which is true. But it would have been another high-scoring, defensive-slippage victory, and those aren’t going to win championships.

The Cavaliers imposed their will defensively in the second half, holding the Celtics to 37.8 percent shooting and just 39 points. Cleveland controls games with its size, and the Celtics expected to struggle scoring. But they have to win games when shots aren’t going down.

They have to find ways to win when they aren’t playing pretty basketball. The second half was a grind, and the Celtics botched their chance to win with turnovers, poor possessions, and a lack of defensive stops.

Mitchell and LeVert combined for 50 points on 14-for-27 shooting after halftime. They got to their spots. They made shots they generally make, and the Celtics did not make things difficult for them. The Cavaliers, unlike the Celtics, made defensive adjustments in the second half, collapsing on Tatum, turning Brown into the primary scorer and luring Smart into shooting more than he should have.

Smart attempted as many shots as Tatum in the third and fourth quarters. In overtime, Mitchell and LeVert scored 16 of Cleveland’s 19 points. The Cavaliers challenged Boston to stop them. The Celtics couldn’t.

Mazzulla postgame explanation didn’t focus on the defensive shortcomings, but rather how they were a couple of plays from winning. Early season is the time to build habits, learn structure, and establish an identity.

The Celtics identity so far is a team that can only win when they are scoring. That likely won’t be the case all season, but it’s the case right now.

“In the second half, we lost our pace, we lost our speed,” Mazzulla said. “We were unable to create separation. We were still in position to win the game if we make winning plays down the stretch. I don’t know if [defense] is as much of an issue as we have to make winning plays. We can’t take anything for granted.”

Defensive stops are winning plays. Doubling Mitchell and LeVert to force the ball out of their hands and toward less prolific teammates is a winning play.

“I thought we had some possessions where we could be better with personnel, our tendencies, individual defense,” Mazzulla said. “They were real comfortable. At the same time, as I said late in the game, if we close it out, rebound, we put ourselves in good position. We have to know personnel. We have to learn how to win again.

“Each season you come into it, you’re not guaranteed to win. It’s value the ball. It’s knowing tendencies and it’s boxing out and rebounding. It’s the fundamentals.”

After Friday’s loss the Celtics are ninth in the Eastern Conference with a minus-1 point differential, which means they’re allowing more points than they’re scoring. That’s what happens when you lose two games by a combined 27 points.

“I’m not super concerned; it’s obviously something we need to work on,” Tatum said of the defense. “Just get on the same page. Obviously, we know the level of defense we can play as a unit and it’s just about finding ourselves and getting back to that place. Even before [Robert Williams returns from injury] we’ve got enough to be a really good defensive team. We’ve just got to continue to make that effort every single play or more often than not than we are right now.”

Returning back to their defensive form from last season wasn’t going to be easy, but it seems the Celtics forgot all their principles in the offseason. Mazzulla has his first major obstacle in his coaching tenure and the Celtics need to adjust soon or risk losing a lot of these shootouts.