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What the heck happened?

A year ago, the Celtics had set themselves up to control the NBA’s Eastern Conference by this point, their 18-3 start seemingly representing a coming of age. The 11-10 start to the 2018-19 season, on the other hand, has made that inspired stretch a distant memory.

So what’s happening? From a statistical standpoint, where are the similarities and differences between this year’s and last year’s performances through 21 games?

Scoring and league context

Scoring is soaring in the NBA. On average, teams are putting 110.6 points on the board, up from 106.3 in 2017-18. Last season, just one of the 30 teams had a scoring average as high as the league’s current average (the Phoenix Suns, with 113.3 per night).

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Related: Gasper: Some Celtics players don’t see themselves as role players — and that’s the problem

Against that backdrop, the Celtics would be expected to both score and give up more points. That’s happening — but the offensive improvement isn’t matching their defensive regression.

Their scoring is up 3.5 points per game, from 103.6 to 107.1, meaning their improvement hasn’t kept pace with the rest of the league. But their defense has seen its allowance rise from 95.8 to 104.3 — an increase of 8.5 that roughly doubles the league average.

Year-over-year offense A look at the Celtics through 21 games this season and last.
Category 2017-18 2018-19
Record 18-3 (.857) 11-10 (.524)
Points/Game 103.6 107.1
FG% 43.8% 44.0%
2-Point FG% 48.2% 49.9%
3-Point FG% 36.0% 35.0%
Effective FG% 50.3% 50.9%
2-Point Att/Game 53.7 54.6
3-Point Att/Game 30.9 35.9
FT% 77.6% 77.5%
FT Att/Game 23.9 19.3
Opponent Fouls/Game 21 20.5
Turnovers/Game 13.3 13.2
SOURCE: Basketball-Reference.com

A timid offense

Though the Celtics have bumped up their scoring average by firing a lot more shots from behind the arc (35.9 per game, 5 more than they averaged through 21 games in 2017-18), numbers back the impression that they have moved the ball around the perimeter without having the playmakers to balance their attack with inside threats.

Boston has made 61.5 percent of its attempts within 4 feet of the basket, meaning that shots from that distance are worth, on average, more than 1.2 points. That’s a high yield. But the Celtics haven’t been able to get to the rim.

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Just 29.4 percent of their shot attempts have come from that distance, the second-lowest frequency in the game. A year ago at this point, they ranked in the middle of the pack, with 34.5 percent of their attempts coming within 4 feet of the bucket.

According to NBA.com, the Celtics average the third-fewest drives to the basket (34.8), and they likewise rank 28th in shot attempts (15.0) and baskets (7.2) when driving to the rim. To this point last year, they averaged 21.8 points off drives (baskets and free throws). This year, that number is down to 17.7 — partly a reflection of the fact that they’re shooting 4.6 fewer free throws per game.

More broadly, the Celtics have shown little ability or appetite to take shots at the rim or short jumpers (4-14 feet, a distance from which they have taken 16.7 percent of their shots – 10th lowest in the league).

Instead, they’ve seemed intent on hoisting 3-pointers (38.1 percent of their shots, third-highest in the NBA) and, when those aren’t available, settling for the least valuable shot in the game: the long-range 2-point jumper, with 15.9 percent of their shots (seventh most in the NBA) coming from 14 feet to just inside the arc.

Shot selection through 21 games* A percentage breakdown of the Celtics' shooting by distance
Distance to basket 2017-18* 2018-19
0-4 feet 34.5 (12th) 29.4 (28th)
4-14 feet 17.9 (17th) 16.7 (21st)
14+ foot 2-pointers 14.2 (20th) 15.9 (7th)
3-pointers 33.4 (10th) 38.1 (3rd)
SOURCE: Cleaning The Glass; * - League ranks through Nov. 25, 2017 in 2017-18, through Nov. 27, 2018 in 2018-19
FG% by shot selection through 21 games
Distance to basket 2017-18* 2018-19
0-4 feet 55.8 (29th) 61.5 (19)
4-14 feet 37.0 (19th) 41.3 (8)
14+ foot 2-pointers 44.4 (3rd) 39.5 (19)
3-pointers 36.6 (18th) 35.5 (20)
SOURCE: Cleaning The Glass; * - League ranks through Nov. 25, 2017 in 2017-18, through Nov. 27, 2018 in 2018-19

Moreover, they’ve been incredibly reluctant to take shots with defenders anywhere near them. Through 21 games last year, the Celtics took the third-highest percentage of shots (13.9) with an opponent no more than 2 feet away; this year, that number has fallen to 10.3 percent, 28th in the NBA.

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They likewise are taking few attempts with defenders within 2-4 feet (31.4 percent, last in the NBA), while ranking in the top two in shots taken with defenders either 4-6 feet or 6 feet-plus away.

On the one hand, that suggests that the Celtics are getting a lot of good looks at the basket. On the other, it means they’re taking a lot of shots that defenders want to give them — and they’re failing to create opportunities in the absence of wide-open shots.

Along those lines, it’s worth noting that Kyrie Irving is the lone playmaker on the team who ranks among the top 50 players in shots per game when a defender is within 2 feet. Jaylen Brown is the only other Celtic in the top 100.

Interestingly, the frequency of open shots taken — and missed — by the Celtics suggests potential upside. While they have taken the highest percentage of shots with a defender 4-6 feet away in the NBA, they have drained just 48.8 percent of such shots — third-worst in the league. They’re leaving some points on the board when getting clean looks; even an improvement to the middle of the pack would boost their scoring in a meaningful way.

Threatened from 3

Last year, the Celtics’ strong start was driven in no small part by their ability to defend every part of the floor. They ranked in the top 10 in the NBA in limiting field goal percentage from every area: shots within 4 feet of the rim, short jumpers from 4-14 feet, longer 2-pointers of more than 14 feet, and 3-pointers.

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But this year, there’s evidence that they are faltering against the two most valuable shots — those closest to the rim and those beyond the 3-point line.

While opponents are averaging only slightly more field goal attempts from near the rim against the Celtics this year (34.9 percent) compared with the first 21 games of 2017-18 (34.4 percent), the Boston defense has been in a relative scramble, resulting in three more free throws attempts (on average) for opponents this year.

The weakened interior defense has freed opponents to attack from beyond the arc. According to Cleaning The Glass, opposing teams are hoisting a whopping 35.3 percent of their shot attempts against the Celtics from behind the 3-point line, the fifth-highest frequency in the NBA.

Opponents are taking seven more 3-point shots per game against the Celtics, with no corresponding dropoff in their ability to make those shots. Indeed, most of that increase in 3-pointers attempted is coming at the expense of the least valuable shots — the long-range 2-pointers, with just 12.0 percent from at least 14 feet but inside the 3-point line, down from 17.4 percent through 21 games last year.

Whereas the Celtics have fired more threes while giving up the interior, opponents routinely have succeeded in getting the defense to collapse with dribble penetration to create open 3-pointers.

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Defense through 21 games* A percentage breakdown of opponents' shooting by distance
Distance to basket 2017-18 2018-19
0-4 feet 34.4 (20th) 34.9 (9th)
4-14 feet 18.9 (20th) 17.8 (17th)
14+ foot 2-pointers 17.4 (20th) 12.0 (11th)
3-pointers 29.3 (9th) 35.3 (26th)
SOURCE: Cleaning The Glass; * - League ranks through Nov. 25, 2017 in 2017-18, through Nov. 27, 2018 in 2018-19
Opponent FG% by distance through 21 games
Distance to basket 2017-18 2018-19
0-4 feet 34.4 (20th) 34.9 (9th)
4-14 feet 18.9 (20th) 17.8 (17th)
14+ foot 2-pointers 17.4 (20th) 12.0 (11th)
3-pointers 29.3 (9th) 35.3 (26th)
SOURCE: Cleaning The Glass; * - League ranks through Nov. 25, 2017 in 2017-18, through Nov. 27, 2018 in 2018-19
Year-over-year defense A look at the Celtics' defense through 21 games this season and last.
Category 2017-18 2018-19
Points/Game 95.8 104.3
FG% 43.5% 44.1%
2-Point FG% 48.5% 51.5%
3-Point FG% 32.5% 32.1%
Effective FG% 48.5% 50.2%
2-Point Att/Game 57.3 54
3-Point Att/Game 26.1 33.1
FT% 75.4% 74.4%
FT Att/Game 19.7 22.7
Fouls/Game 19.8 21.4
Turnovers/Game 14.9 15.5
SOURCE: Basketball-Reference.com

Sluggish at the start

Prior to this week’s win over New Orleans, the Celtics lamented their recent rash of sluggish starts — a development that had run counter to what they’d done through the first few weeks of the schedule.

The Celtics average 25.4 points in the first quarter, the third-lowest average in the NBA. However, they’re limiting opponents to 24.0 points in the opening 12 minutes, the lowest yield in the NBA.

Yet those season-long numbers fail to account for a more recent downturn. Through the first 10 games of the year, the Celtics had absolutely smothered opponents in the first quarter, holding them to 20.9 points — easily the lowest mark in the NBA. But in the last 11 games, the Celtics have given up 26.9 first-quarter points while averaging 25.0.

That shift has left the Celtics routinely playing catch-up, a pursuit in which they haven’t been terribly successful. They are 3-7 when getting outscored in the first quarter.

More Celtics coverage:

■   Finn: A look at each Celtics player and how they’re contributing to a bewildering start

■   The problem with starting Marcus Smart is you can’t bring his energy off the bench,’ said coach Brad Stevens

■   Washburn: Marcus Smart’s impact is evident — keep him in the Celtics’ starting lineup

■   Washburn: What’s wrong with the Celtics, and how to fix it


Alex Speier can be reached at alex.speier@globe.com. Follow him on twitter at @alexspeier.