The Celtics are the defending NBA champions, and still the current favorites to win it all again this season.

But since the opening week of December, Joe Mazzulla’s club has looked decidedly ordinary.

Boston’s 118-114 home loss to the Philadephia 76ers on Christmas Day was its second straight and fourth in seven games. It was the Celtics’ first losing streak since last season and their first time dropping four in a seven-game span since the 2023 Eastern Conference finals against Miami.

Their three wins were blowouts over Detroit, Washington and Chicago. The Pistons and Bulls are fringe playoff teams at best in a watered-down Eastern Conference, and the Wizards are the worst team in the NBA. At 22-8, the Celtics now sit closer to the third-place New York Knicks than to the East-leading 26-4 Cleveland Cavaliers.

After Wednesday’s holiday loss at TD Garden — where the Celtics already have lost more times (five) than they did all of last season (four) — Mazzulla said they are playing “inconsistent basketball right now.” His players agreed, acknowledging their performance of late has not been championship-caliber.

“I think we’ve got to take some ownership,” Jayson Tatum said. “We’ve got to be better. We’ve got to acknowledge the things 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 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.”

The Celtics trailed by double digits in each of their last four losses. Against Philly, they erased a 16-point deficit, only to fall behind again by 15 midway through the fourth quarter. They nearly overcame that gap, as well, but couldn’t close in the final minutes.

Since Dec. 7, the Celtics rank 11th in offensive rating, seventh in defensive rating and eighth in net rating — respectable marks, but well off their pace from last season, when they were first, second and first, respectively.

“To Joe’s point, he said there’s some inconsistencies,” Al Horford said. “But ultimately for us, it’s figuring things out on the defensive end, offensive end, continuing to work, continuing to try to get better. That’s the goal there. So that’s my mindset at this point. We don’t like losing. We don’t like losing here at home, especially. So we just have to be better.”

Horford did raise a valid point about this Celtics season so far. Thirty games in, Boston has yet to play a full game with its entire core rotation (Tatum, Jaylen Brown, Derrick White, Jrue Holiday, Kristaps Porzingis, Horford, Payton Pritchard and Sam Hauser) all available. All eight have been active on the same night just once: a Black Friday win at Chicago during which White exited with a foot injury.

At this point last year, the Celtics’ top eight had missed a total of 20 games, with only Porzingis sitting out more than four. That group has missed 46 games this season, including five-plus by Brown, Horford, Hauser and Porzingis.

“For us, we’re figuring out being in this position,” Horford said. “One of the things that doesn’t get necessarily talked about: Last year, I felt like we were pretty healthy throughout. And I think this year has been a lot of in-and-outs and guys filling in and guys doing this and doing that. So there’s different dynamics. It’s just different. I just think we continue to figure it out. Defensively, continuing to make strides there. And then on offense, making sure that we’re playing a style of ball that we want to play, which is playing faster and move the ball, and that type of play.”

On Wednesday, Holiday sat with a shoulder injury and Porzingis missed the second half with what the team called left ankle soreness — the third injury he’d suffered since returning from offseason surgery on Nov. 25.

“It always feels a little bit worse than what it actually is,” said Brown, who was held to two points on 1-of-8 shooting in the first half against the Sixers before scoring 21 after halftime. “We’ve got some different guys in the lineup and stuff like that. So I feel like we haven’t had as much continuity as we would like when we’ve been out there. So we’ll just take a look at it, and it’s a moment where we can improve as a team.”

Improvement will be necessary as Boston now enters a tricky stretch of its schedule. Six of its next seven games are against teams that entered Thursday at or above .500, beginning with a home-and-home against an Indiana Pacers squad that hung 135 points on the Celtics back in October.

Speedy opponents like the Pacers can cause matchup problems for the C’s, who have lost to the teams currently ranked first (Memphis), second (Atlanta), third (Chicago), sixth (Indiana) and seventh (Cleveland) in pace this season.

Boston also is closing in on its first Western Conference road trip: at Minnesota on Jan. 2, Houston on Jan. 3, Oklahoma City on Jan. 5 and Denver on Jan. 7.

The Thunder and Rockets own the two best records in the West and the top-ranked defenses in the NBA, and they went a combined 11-1 between Dec. 7 and Christmas Day. The Timberwolves lost to the Celtics by two at TD Garden on Nov. 24, and the Nuggets, whose superstar centerpiece, Nikola Jokic, is the early front-runner for NBA MVP, beat Boston twice last season.

“We’re not panicking or anything,” Tatum said. “We’ve just got to man up and look in the mirror and figure out some things that we’ve got to do better at. Everybody’s fully capable, and we’ve always done a really good job of responding, and I have no doubt that we will. So not panicking, but we’ve got to be better, and we will.”