The Celtics arrogantly overlooked the Suns and got humbled

Taking the lead was just out reach for the Celtics over the final three quarters.Barry Chin/Globe Staff

It seems like coach Joe Mazzulla could have held up a “beware” sign as the Celtics entered the tunnel for Friday’s game against the Phoenix Suns and the same thing would have happened.

The Celtics again took a lesser, shorthanded opponent lightly, thinking jumping out to a 7-0 lead would discourage another professional basketball team, one just two years removed from the NBA Finals. It didn’t.

They allowed Phoenix, missing two starters, to get comfortable and then spent the whole game chasing. A team with the NBA’s best record lost again to a team it should have easily beaten.

The 106-94 loss may not have long-term repercussions but it does revive the issue of whether the Celtics can consistently win when threes aren’t falling and the offense is choppy. Jayson Tatum made just three field goals, his fewest since Nov. 28, 2021, a stretch of 104 games.

Al Horford and Grant Williams combined for three points in 52 minutes. Malcolm Brogdon was a minus-28 in his 22 minutes.

The Phoenix Suns are cohesive and well coached, but they also were embarrassed by 32 points at home Wednesday by Atlanta and were missing Devin Booker and Cam Johnson. The Suns beat the Celtics not with prolific scoring but with a paper-cut method, scoring from a bunch of different sources, players who capitalized on their uncommon opportunity to play extended minutes.

Phoenix was excited about this chance to face the NBA’s best. The Celtics treated it as a summer league game. That’s disrespectful and they paid the price.

“The way that we played, that’s fair to say,” Horford said when asked if they took the Suns lightly. “We have to find a way. Today, you let a team get confidence like that and it’s tough to slow them down. They’re professionals and we need to do a better job with that despite who is on that other side. There’s a certain level we need to play with.”

The Celtics have enough talent to win the championship, although they could use some roster reinforcements over the next few weeks. But their mental toughness will come into question after nights like these. The NBA has become a league where any team can lose on any night, and every club has its share of clunkers.

But the Celtics lost Friday because they didn’t hustle enough, they didn’t push the pace, they looked bored at times. They lost because the bench was trounced by their Phoenix counterparts. They needed to bring energy and they failed miserably.

“We can’t be inconsistent in how we execute,” Celtics coach Joe Mazzulla said. “I thought we had a ton of empty possessions offensively and that’s when we kind of were just like, [we got discouraged]. We have to find a different way to execute.

“I don’t know if it’s a toughness thing as much as it’s a math thing. You can’t get outshot by 20. You can’t have 15 turnovers. You have to be able to win on those margins all of the time.”

The Celtics’ recent skid — four losses in six games — has coincided with the absence of Marcus Smart, who missed his fifth consecutive game with an ankle sprain. And while he impacts the offense with ball movement, the Celtics have enough depth to consistently win without him.

Injury can’t be and shouldn’t be an excuse here. Every other club deals with injuries and the Celtics have been fortunate enough to keep their core reasonably healthy for most of the season. The issue is mental and the Celtics’ approach when the opponent isn’t considered formidable.

“They outplayed us, no energy, they got more shots than we did,” guard Jaylen Brown said. “We’re walking the ball up the floor every possession. We just didn’t have the enthusiasm tonight. Just kind of going through the motions as a unit. We’ve got to be better. We know that and tonight was a poor example of that. We didn’t come out with the right energy and we got beat.”

The admission of going through the motions is quite an indictment on a team that swears it has learned from last season’s NBA Finals experience. Losing to an undermanned team is one thing, but then there is a lack of focus, a lack of intensity. When you allow reserve players to get comfortable early because of lack of knowledge or respect, that’s a serious concern.

It’s something the Celtics were supposed to have learned last month against Oklahoma City, or the three times they’ve lost to Orlando, but playing down to the level of the opponent continues to be a weakness for a team that is now clinging to the No. 1 spot in the Eastern Conference.

The remaining 29 games will be filled with difficult opponents — three games against Philadelphia, two against Milwaukee, two vs. Cleveland, two against the Knicks, Memphis — that the Celtics can’t slip against these shorthanded or lesser talented opponents or it could cost them in the long run.

The question now is whether Mazzulla and the team leaders change their approach in these situations or continue to be humbled because of their arrogance.