Arrogant Celtics aren’t holding up bargain on other end of the court

The NBA is a humbling league, and the Celtics glided through their first five games without feeling truly humbled. They lost two of those games — a rugged game at Chicago in Dwyane Wade’s home debut and then a shootout at Cleveland without two starters.

Even without those two starters again Sunday night the Celtics took their opponent for granted. There was no way the overshadowed, lightly regarded Denver Nuggets would come to TD Garden and punch the home team in the mouth, especially after looking so uninterested the night before in Detroit.

Arrogance has crept into the Celtics’ locker room. They believe they are the second-best team in the Eastern Conference. They believe they are good enough to beat opponents without much of a defensive effort, and that is disturbing after just six games.

The Nuggets scored a mind-boggling 77 points in the first half and cruised to a 123-107 win, as the Celtics have now allowed 251 points in the past two games.

The words “elite,” “staunch,” and “dominant” were thrown around to describe the potential of the Boston defense in the preseason. Those should be replaced by “putrid,” “awful,” and “miserable.” The Celtics allowed Denver second-year guard Emmanuel Mudiay, who scored more than 20 points in just 10 games last season, to tally 24 points in the first quarter.

Mudiay had missed 51 shots through his first five games but yet managed to make 9 of 10 in the opening quarter. The Nuggets scored in an array of ways against the soft Celtics defense as they converted 64.4 percent of their first-half shots.

They also capitalized on the Celtics’ laziness by drawing 14 first-half fouls. The Celtics are reaching too much and then trying to swipe away when they’re beat off the dribble. It’s a byproduct of complacency; the Celtics have grown fond of themselves for some reason.

They shouldn’t be so fond anymore.

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“The nicest way I could say this is I think we play like a finesse team and [the Nuggets] play physical,” Celtics coach Brad Stevens said. “So I kind of saw that right out of the gate. They were getting where they wanted to on their drives. And that’s who we’ve been the last week.”

Stevens didn’t want to say “soft,” but he basically did. Without Jae Crowder and Al Horford, the Celtics have lost their defensive identity and are allowing opponents free reign at the rim without repercussions. They aren’t even fouling hard. They are slap fouls. They are not putting anyone on their backs.

So how is that method working so far? The Celtics are 3-3 and are allowing nearly 112 points per game. They are imposing no physical will on the game, and when they do, they foul. The Celtics attempted 119 free throws in six games. Their opponents? 182.

They have been outscored by 54 points this season at the free throw line, or nine per game, which puts even more pressure on the offense to be precise. The Celtics’ offense has been solid but hardly good enough to compensate for their Swiss cheese defense.

Isaiah Thomas responded with a little Dennis Green, the deceased former NFL coach.

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“We’re not as good as we thought we were,” Thomas said. “When we don’t come out with a sense of urgency, we’re not a good team. That’s what happened. At this point, we’re all talk. That’s all we’re doing. Until we put it together in a full 48-minute game, show what we can do on that end, we’re not going to be a top defensive team.”

At this point, just being slightly resistant would be a start. Allowing 77 points in 24 minutes is pretty difficult to do, especially against a team without an elite scorer. If that team has LeBron or Curry or Harden, sure, they can get hot, score 15 in a row, drain some difficult 3-pointers.

But Mudiay can’t be allowed to score 24 points in 12 minutes, with many of those shots open jumpers. It’s embarrassing. Just like Cleveland can’t be allowed to convert 11 first-half 3-pointers and score 68 in the first half. The Celtics pride themselves on their depth and it’s not helping with Crowder and Horford sidelined.

Stevens took responsibility for the defensive downfall in sort of a parental way, meaning he blamed himself for allowing the players such freedom to police themselves.

“I told the players it’s my fault because I watched the players play this way,” he said. “The bottom line is, when you have a spot you have to hold it. When you have a spot you have to take it. Whether it’s changing who plays, whether it’s creating a new scheme, we just have to figure that out.

“I thought this was possible.”

But in reality, it’s up to the players to assert themselves. They have to be insulted and embarrassed when a 27 percent shooter scores at will. They can’t simply believe they are good because someone else says so. This is a humbling league and with the NBA’s parity, teams are going to spend this season slapping each other around.

BOX SCORE: Nuggets 123, Celtics 107

Gary Washburn can be reached at gwashburn@globe.com. Follow him on Twitter @GwashburnGlobe.