'Inconsistent'. 'Too casual.' 'Need to man up': Celtics can't afford to ...

Celtics "We've got to take some ownership," Jayson Tatum said. Luke Kornet and Jayson Tatum react to a call they don't like. Erin Clark/Globe Staff

The Celtics let a division rival beat them on their home floor in front of a national TV audience on Christmas Day.

It’s not like the Sixers (11-17) have been dominant this season. They have regained some of their swagger, winning four of their last five behind the stellar play of Tyrese Maxey, who dropped 32 points or more in three of those contests.

There’s a lot of basketball left, but if the season ended today the Sixers wouldn’t even be in the Play-In Tournament. The effort that the defending champs turned in on Christmas was unacceptable, with players saying as much in postgame press conferences.

“We’ve got to take some ownership,” Jayson Tatum said. “We’ve got to acknowledge some things that we’ve done that are not so great. We’ve got to look in the mirror and man up. We’ve just got to be better.

“We fully believe in ourselves, the things that we can do when we’re fully locked in. We’ve done it time and time again. We’ve had some lapses recently and we’ve just got to get back on track.”

The Sixers were healthy, but the Celtics’ other lapse this week came against the Magic on Monday, who were missing three of their best players. With the Christmas Day loss, the Celtics now own their first losing streak of the season.

The Celtics (40.8 percent) actually had an above-average shooting night from 3-point range. It was the little things that hurt the them in this loss. They didn’t hit free-throws, shooting 61.5 percent as a team from the charity stripe. Philadelphia was a perfect 19-for-19.

They took too long to display the proper effort on either end of the floor, looking sluggish and not fully engaged in the first half.

Having Kristaps Porzingis limited to 13 minutes after tweaking his ankle in the first quarter certainly didn’t help. The 7-foot-2 center tried to play through the pain, but was clearly hobbled for several minutes in the opening stretch.

The Sixers took advantage by turning the tempo up, and the Celtics simply did not look ready to meet the challenge. Jaylen Brown said the effort was lacking. Effort is the one thing you always can control no matter what is going on around you.

“I just think we came out too casual,” Brown said. “I just think we were walking to our spots. Nobody was sprinting down the court, just kind of hanging around, just trying to veer back to get the ball instead of just pushing it down the court and being aggressive. I feel like we started off the game slow and they took advantage of that.”

The offensive rhythm just felt off, both Brown and coach Joe Mazzulla said. The Celtics were able to clamp down on Philadelphia defensively in the third quarter, allowing just 16 points.

Playing catch up for the whole night is not a recipe for success.

“We’re playing inconsistent basketball, so we’ve got to be better on both ends of the floor, be more consistent on both ends of the floor,” said Mazzulla.

Thirty games into the season, the Celtics are still in a pretty solid spot. They’ve won 73 percent of their games. They’re second in the Eastern Conference. They’ve managed a variety of injuries that didn’t seem as frequent last season.

But there has been some slippage compared to last year, and it’s been especially noticeable this week.

It’s a long season and nights like this happen, but they can’t be allowed to happen consistently. That’s where effort, discipline, and coaching come in. These kinds of games must remain abnormal if the Celtics want to raise Banner 19.

“I think that’s a fair criticism from him,” Al Horford said of Mazzulla’s comments about the inconsistent play. “I just think we have to really dive into our work a little more to understand that we can’t relax during periods of the game no matter the circumstance. We have to make sure we’re better in that regard.”