Since the middle of December, the Celtics rank in the top five in the NBA in offensive rating, defensive rating and net rating. They’re fifth in points scored per game during that span and sixth in points allowed.

A team that’s excelling in all of those categories should, it stands to reason, also own one of the league’s best records. But Boston does not.

During that 21-game sample, the Celtics have hovered just above .500: 11-10 entering Wednesday night’s game against Chicago. Twelve teams have a better winning percentage. Overall, Boston sat at 32-15 entering the Bulls matchup, 5.5 games out of first place in the Eastern Conference and just three losses shy of their total from all of last season.

What’s the problem? Why are the defending champions posting enviable stats but struggling to stack wins? Why has their six-week swoon featured just two winning streaks, neither longer than three games?

In Joe Mazzulla’s eyes, it comes down to late-game execution.

“To me, in this league, the difference between winning exponentially more than not is close games, and we’ve lost a couple of those in different ways,” the Celtics coach said before Wednesday’s game. “That can be the separating factor between a five-game win streak and losing three out of five, is how you close out the last two minutes of the game.”

The numbers and the eye test bear that out. During that same span, the Celtics’ defensive rating in clutch situations (the final five minutes of the fourth quarter or overtime when the score is within five points) tumbles to 26th in the NBA. They’ve blown nine fourth-quarter leads this season, most recently coughing up a 12-point advantage in the final nine minutes of Monday’s last-second home loss to the Houston Rockets.

Boston has lost five of its last eight games that were decided by five points or fewer, with one of those wins coming in overtime against a Los Angeles Clippers team that rested nearly its entire rotation. Several of the losses resulted from costly turnovers, fouls and defensive lapses in the final minutes.

The Celtics’ struggles haven’t been limited to crunch time, however. The entire fourth quarter has been a problem area for Mazzulla’s squad. Boston’s fourth-quarter rankings since Dec. 16: 18th in offensive rating, 20th in defensive rating and 22nd in net rating.

Last season’s Celtics led the NBA in fourth-quarter net rating and had the fourth-best net rating in clutch situations. Regaining that composure in time for the playoffs will be critical.

“It comes down to close games, and you see those all the time,” Mazzulla said. “The difference between winning and losing is very small. You’ve got to fight to get those details. You can have a really good process over the course of a game or 10 games, but I think closing out games is the difference between, sometimes, eight to 10 extra wins in a season.”

Porzingis agreed with tech reversal

The NBA rescinded the technical foul Kristaps Porzingis received for dunking on and then bumping the Rockets’ Dillon Brooks on Monday night. Unsurprisingly, the Celtics center believed the league made the correct decision.

“Honestly, as I got the putback, I didn’t even see (Brooks) there,” Porzingis said Wednesday. “I was just running back and there was some contact. I didn’t think — there’s different angles of the cameras. Some look worse; some look better. But honestly, you could see as I was running, I wasn’t looking at him where I’m looking for him. He was just in my way, and he got bumped, and that was it. I think it was fairly rescinded.”

Porzingis also shared his thoughts on the Celtics’ lack of consistency, which dates back to mid-December. He believes Boston has been especially unlucky of late — he noted the alarming number of opponents, including Brooks, who have enjoyed season-best 3-point shooting nights against the C’s — and reiterated his stance that the team will eventually stabilize.

“The luck hasn’t been on our side for the most part of the season,” Porzingis said. “I just saw a tweet also of the amount of players that have set their 3-point records just this year, just 2025. So I think at the end, everything will even out a little bit and we’ll go on a nice run. And as I keep saying, we’ll peak at the right moment.”

Off the rim

Derrick White returned to action Wednesday after missing Monday’s game with a shin contusion. Jayson Tatum also was active after being listed as questionable with right knee tendinopathy. Al Horford (big toe sprain) and Sam Hauser (hip impingement) both sat out their second straight game. … Chicago was without leading scorer Zach LaVine, who was inactive for personal reasons. … Mazzulla praised third-year Celtics guard JD Davison, who was selected play in the 2025 Rising Stars game during NBA All-Star weekend. The coach compared Davison’s year-to-year development — which has mostly occurred with the G League’s Maine Celtics — to Payton Pritchard’s. “Every year, he adds something to his game,” Mazzulla said.