There’s a perception the Boston Celtics have long been a great third-quarter team. But after another game blown following halftime, that reputation is fading this season.

Looking at net rating in the third quarter, the Celtics have generally been toward the top of the league in the regular season. But then in the playoffs, it tends to fall steeply. Over the past four years, they’ve ranked second, first, ninth, and first in the regular season, then ninth, 10th, 14th and eighth out of the 16 playoff teams. They’ve gone from elite in the regular season to having a negative net rating in the playoffs.

So what ails the Celtics in third quarters? Part of it is the way they come out of the locker room, often ceding some ground in the opening minutes.

Looking at the first rotation of the third quarter from the Indiana loss Monday, things went fairly smoothly. There was some tremendous shot-making by Jaylen Brown and Tyrese Haliburton, the Indiana Pacers took advantage of a few switches, but it was pretty even basketball. But when Derrick White blocked Haliburton and the Pacers star collected his miss to score and take the lead, that’s when things started to go south, with the Pacers playing some of their best basketball of the season.

The next play ended with Brown missing a mediocre post shot, then Haliburton pushing the pace. He got by White, caught Luke Kornet in an awful position, and got right to the rim to open up a two-possession lead. When White said he was horrible in the second half, he was talking about plays like this.

Typically, that starting core opening up halves executes solidly, even if they give up a chunk of the lead. But then things get messy when the second unit comes in. It often is placed on Brown’s shoulders, but he’s been playing well in these minutes for the most part. The problems come from how few ball movers are on the floor.

Just watch what happens when the Pacers switch this stagger screen perfectly and then Boston’s bench players don’t know what to do next.

White has to get rid of the ball on his drive to kick it to Dalano Banton; he misses Brown who is open up high and drives to nowhere. White then misses the chance to drive over a Brown screen to step into an open 3 and bricks the spot-up instead.

The next few minutes were a mess for both teams, but Sam Hauser’s persistent crashing on the offensive glass resulted in him hitting a relocation 3 to make it a one-point game. Then this happened.

this footwork from T.J. McConnell 🤌 pic.twitter.com/SBKSjZ2blp

— Indiana Pacers (@Pacers) December 5, 2023

These are the kinds of plays that have defined the third quarter second unit. Players like Kornet will play well in the first half, then the opponent figures out how to work around them and starts to find some openings. At Kornet’s size, there’s no way a small guard should be able to twist around him that far from the hoop.

To Kornet’s credit, he learned his lesson a few possessions later and swatted another T.J. McConnell shot from this spot like he was guarding his little brother in the driveway. Of course, that ball fell right to Buddy Hield, who buried a 3. Boston gave up five points in this stretch because they blocked a shot that fell right to an Indiana player. That’s terrible luck.

After Kornet was called for a moving screen on the next play, the Celtics broke out this bizarre coverage where Kornet started the play covering point guard McConnell. When McConnell drives over a staggered screen, Kornet rolls underneath the whole coverage as Payton Pritchard switches onto the ball. That leaves Jrue Holiday stuck between two shooters on the weak side and it leaves nobody in position to close out to Bennedict Mathurin open in the corner.

Credit to Hauser for being proactive, but the Celtics looked confused there. That carries over to the offense, where the second unit can often become incredibly Brown-centric in the half court with mediocre spacing. Just look at where Pritchard is standing on this Brown drive.

When Brown meets resistance trying to hit the lane, he looks to the wing for a Pritchard outlet. But Pritchard is standing out by the logo straight behind Brown, making for a difficult pass that gets picked off with ease. If Pritchard had slid down to the wing, following this green line below, the Celtics could have moved ahead of the defensive rotation.

That capped off a 17-3 Pacers run that blew the game wide open. These are the kinds of plays that show where Brown can struggle to play under control and recover his dribble to a safe area. But it also reveals how he is sometimes working with players who don’t strike fear in the defense or aren’t in the right spots.

A couple of weeks ago, we explored how he was making good passes to find teammates wide open in the third quarter, but they were missing a lot of those shots. The issue then came in the fourth, where he locked into scoring more and would miss some of the open looks he could create for his more talented teammates.

The Athletic’s Shams Charania was the latest to report the Celtics will be looking to acquire bench depth, something that’s been presumed since the Celtics entered the season with Pritchard, Hauser and Kornet as their main bench pieces after Al Horford.

While Horford continues to age like he’s living in a blue zone on the coast of Sardinia, you can see age naturally sapping a bit of his explosiveness when he’s guarding switches or trying to attack open space.

He didn’t even try to drive the gap on this first play, putting White in a tight corner for the shot. Then on the second drive, his lift just wasn’t there. In the past, Horford would conserve his energy most of the night and then show some athleticism when the rim opened up, but that’s not quite there right now and/or anymore. I’m legally mandated to remind you that presuming Horford is washed up has time and again proven to be foolish.

Horford has been a bit hit or miss on switches, but credit to him for moments like at the six-minute mark of the fourth where he stayed in front of Haliburton on a baseline drive. Unfortunately for him, Jayson Tatum left his feet on the closeout and Aaron Nesmith was able to get to the rim. Boston’s defensive cohesiveness sometimes gets exposed by teams who play with a lot of speed and ball movement.

But bring Kristaps Porziņģis back in the fold, put Horford back into his reserve role, and suddenly you take away some of those big defensive mismatches that hurt the second unit defense when Brown is out there. They get more stops, Brown can attack more in transition, and their half-court playmaking deficiencies are better mitigated.

But now that Porziņģis has dealt with two mild lower-body injuries since joining the team, it’s clear they need more reinforcements. Picking between Kornet and Neemias Queta isn’t going to solve their second unit issues when they need to tap into their center depth. Pritchard showed a little more confidence to attack the paint in Indiana, but his scoring deficiencies this year have been brutal for the team’s depth.

Pritchard brings a ton of activity on defense and he rarely turns it over. His corner crashes on the offensive glass have been a game-saver at times. But they can’t have Hauser being the only scorer off the bench and they need someone who can create offense. They should be patient with Pritchard, but coach Joe Mazzulla will have to give Svi Mykhailiuk a shot at some point since he can run some pick-and-roll and score in a first action off the bounce. But the Celtics look like they need a little bit more if they’re going to hold on to their leads throughout the game in high-level competition.

The In-Season Tournament was exciting because it gave teams like the Pacers their first big stage to show the program they’re building. But it also was a good litmus test for the Brad Stevens front office to figure out whether it needs to keep pushing chips in to improve their championship odds.

It’s just one loss, but it was emblematic of the few shortcomings this team had on paper coming into the season. They have the assets to make something happen. The more the season goes on, the likelier a move seems to be coming.