BOSTON — Brad Stevens wasn’t happy on Sunday evening. The Celtics coach had watched his team get shredded 123-107 by the visiting Nuggets, who were far more physical and made Boston look, in Stevens’ words, “like a finesse team.” That was not a compliment for a team that established itself last year by ranking fourth in defensive efficiency, a number that has plummeted to 29th this year.

Obviously, six games does not make a season, and the Celtics had some lax defensive stretches last season. Also, Boston has not been fully healthy, with Al Horford dealing with concussion symptoms, Jae Crowder out with a sprained ankle and Marcus Smart missing the first three games. It’s not panic time.

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But Stevens said, with the Celtics having yielded 251 points in its previous two games, the goal of being a top three defensive team is far from his mind. “I don’t even know if it’s statistically possible, based on our week,” he said. “You know? I don’t even know if you can make it up in 76 games.”

The Celtics won 48 games last year, and Stevens conceded that some of what he is seeing is probably the predictable result of expected success, the Celtics now mentally expecting to win games without needing to put in the effort.

“I thought this was possible,” he said of the disappointing defense. “I think anytime that you have a year where you achieve relative success on one end of the floor, there’s slippage that’s bound to occur because shortcuts are bound to be taken.”

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After the game, an Eastern Conference scout shared some of Stevens’ worries — that despite the injuries and the fact that it’s only early November, the Celtics have shown some bad habits and perhaps even roster flaws. He pointed out three areas of particular defensive concern for Boston, and we crunched some numbers to illustrate his points.

Pick-and-rolls

Scout says: “They were a top 10 team defending the pick-and-roll last year, but they don’t seem to have their footing at all in those situations this year. There are a lot of players out of place, perimeter defenders are not steering the ballhandlers out of the paint and that leads to a lot of poor decisions from the big guys and that leads to fouls. So it’s the guards, not just the big guys. I don’t think getting Al Horford back automatically solves that, either, because he is not a great defender against the pick-and-roll. They should be a lot more consistent with this.”

The numbers: The difference between last year’s pick-and-roll stats and this year’s are not tremendous, but they add up. Last season, more than one-fifth of the plays the Celtics saw came in pick-and-rolls, and this year, that number is up to more than a quarter. When defending a ballhandler on the pick-and-roll, the Celtics allow 0.84 points per possession (20th in the NBA), up from 0.79 last year (ninth). It is worse when it comes to defending roll men. Boston allowed 0.97 points per possession last year (sixth), but 1.11 this year (23rd).

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There have been fouls galore for the Celtics, who allow 30.3 free-throw attempts per game, 25th in the league. But that isn’t too much different than how they played last year, when they allowed 26.0 free throws per game, 28th in the league. The issue for Boston is that all that fouling is not stopping teams from getting to the rim and finishing.

Spot-up shooting

Scout says: “I don’t see them closing out very hard, I think that is an effort issue. They’re a small team and if you’re going to be undersized, you have got to be able to close hard to the perimeter, and I do not see them doing that the way they did last year. Avery Bradley still does it. He is definitely the leader on that end, and his effort never changed. Jerebko has been good, too. But the rest of them, even Crowder when he was in there, just have not been closing out the way they did last year. So if you can draw them into the paint or set a solid screen, you’re probably not going to get a hard contest and you wind up with an open shot.”

The numbers: The Celtics have been an easy team to beat with jumpers and have been unable to contain drive-and-kick point guards. You get a clear look over their attempts contest outside shots, which are not only coming from relatively small guys, but from guys who mostly need to be giving more effort.

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The Celtics ranked third in spot-up defense last year, allowing 0.92 points per possession. This year, they’re allowing 1.08 points per possession, just 27th in the league. Opponents attempted 23.3 3-pointers per game last year, and made only 33.6 percent, which was fourth. Thus far, they’re allowing 29.7 attempts and 37.6 percent made, which is 26th in the NBA.

Size

Scout says: “I think they overlooked the need for some size you can bring off your bench defensively on the wing when they were building the roster. You know, you’re building a team that can spread the floor and play small on the offensive side, but you have to account for how that hurts you in matchups defensively. I think they have to address that by the (trade) deadline if they want to make the most out of this season. Does not need to be a star, but they need a wing who can hold up well enough offensively and do the job defensively off the bench—a guy like a P.J. Tucker from Phoenix, if he’s healthy, just as an example. They do not have that guy.”

The numbers: The Celtics are the smallest team in the league, though they’re just 14 total inches off from the median height of an NBA roster (which is 1,170 inches, according to FiveThirtyEight). That’s not the whole story, though, because the size deficit is so concentrated on the perimeter.

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The Celtics’ main backcourt options are Isaiah Thomas (5-9) and Avery Bradley (6-2) as starters, with Terry Rozier (6-2) and Marcus Smart (6-4) behind them. For the other 14 Eastern Conference teams, the average point guard is 6-2 1/2, and the average shooting guard is 6-6 — nine teams have point guards 6-3 or taller, and eight start shooting guards 6-6 or taller.

Boston was undersized last year, of course, but they replaced one guy with size — Evan Turner, who is 6-7 — with Gerald Green (also 6-7), who is not much of a defender. The size deficit can be covered up with effort, but Boston may need to add a defensive-minded wing at some point before the postseason.