The Celtics are in a slump — and their biggest issue is surprising

Struggle, in the NBA, is relative. Given the chance to swap their troubles for the ones facing the Boston Celtics — the defending NBA champions, the winningest team in the association over the last half-decade, the owners of the NBA’s third-best record and net rating heading into Friday’s matchup with the Magic — you’d imagine that just about every team in the league, save maybe Cleveland and Oklahoma City, would take the trade without a second thought.

But while teams (and fanbases) might experience struggle on a sliding scale, the same goes for success. After steamrolling through the regular season and playoffs en route to Banner 18, the standard in Boston is sky high; over the last month, the C’s haven’t consistently met it.

After starting their title defense 21-5, the Celtics are now 7-7 over the last month — and one C.J. McCollum near-buzzer-beater away from being 6-8, on the franchise’s first three-game losing streak since the 2023 Eastern Conference finals. They’ve been more or less fine in the aggregate: ninth in offensive efficiency, seventh on defense, outscoring opponents by 5.2 points per 100 possessions outside of garbage time over the last month, according to Cleaning the Glass. But they’ve been more erratic, especially late in games, ranking just 24th in net rating in the fourth quarter over the past month, getting outscored by 4.8 points-per-100.

The Celtics have lost four of their last seven games in the friendly confines of TD Garden; they lost just four regular-season home games total last season. And in the last two weeks alone, they’ve suffered double-digit defeats at the hands of the Thunder (understandable!), the Kings without De’Aaron Fox (hmm, seems weird, but hey, new coach bump?) and the Raptors (wait, what?).

“We’re going through some s*** right now,” MVP candidate Jayson Tatum told reporters on Wednesday after a 110-97 loss to a frankly brutal Toronto team — a listless performance that center Kristaps Porziņģis said featured “no spirit” and “no personality,” and that has become par for the course for the champs of late.

To some degree, you can chalk games like the close shave against the Pelicans or the dispiriting losses to the Kings and Raptors up to the time-honored truism about the wearying challenge unique to champions — of having the bullseye on your back every night, of getting every opponent’s best shot, of how much more difficult it is to do the little things to stay on the top of the mountain after finally reaching the summit.

“I think we’ve got to take some ownership. … We’ve got to look in the mirror and man up,” Tatum recently said. “We’ve just got to be better. We fully believe in ourselves, the things we can do when we’re fully locked in, and we’ve done it time and time again. We’ve just had some lapses recently, and we’ve just got to get back on track.”

Tatum offered those remarks after the Celtics’ Christmas Day loss to the 76ers. Watching the losses to Sacramento and Toronto, though, it seemed safe to say that the C’s have not fully restored their level of lock-in:

Over the full season, only the Rockets, Magic and 76ers have more consistently and thoroughly won the possession battle than Boston. That has actually held up over the past month, with the Celtics continuing to outperform opponents on the offensive glass, commit fewer turnovers than they create, and generate more free throws than they allow.

Lose that battle in an individual game, though — by allowing Toronto to rebound 32% of its missed shots, or committing eight live-ball turnovers against New Orleans, or losing the turnover and offensive rebounding battle so decisively that Sacramento winds up taking 18 more shots — and your margin for error vanishes, and you’ve got to win beyond the arc. Have a hard time doing that, and a 60-win team can start to look like a .500 team pretty damn quick.

And there’s the rub: While better effort and attention to detail obviously never hurts, the issue likely has less to do with insufficient focus and more to do with the same problem and solution that the C’s used to bludgeon the league last season. It is — surprise! — the 3-point shooting.

The Celtics led the NBA in points scored per possession as of Dec. 15. They’re ninth since, a drop-off of 5.7 points per 100 possessions — the fourth-largest decline in the NBA in this span. The only offenses to fall off more precipitously? A Nets team that traded away two starters; a Mavericks team that’s been without Luka Dončić and Kyrie Irving for much of that stretch; and a Heat team that’s spent nearly that entire period in a staring contest with its best player.

The driving force behind the downturn? A monthlong reversal of fortune from distance.

Deep trouble

The Celtics shot 37.2% from deep through their first 26 games, 12th in the NBA. That itself represented a steep descent from last season’s 38.8% mark, but one that Joe Mazzulla’s squad mitigated by cranking up the long-range volume and ripping off the knob, with Boston on pace to become the first team in NBA history to take more than half of its shots from beyond the arc. During this .500 stretch, though? Boston has splashed just 34.2% of its triples — 22nd in the NBA…