A loss to the Knicks was the low point for the Celtics. Here’s how they rebounded into a title contender.

The Knicks defeated the Celtics, 108-105, Jan. 6 in New York.Al Bello/Getty

When asked about their dramatic turnaround, one that has them in the Eastern Conference semifinals and the betting favorite to reach the NBA Finals, the Celtics brass unanimously points to a Dec. 31 home win against the Suns.

Without Jayson Tatum, who was out with COVID-19, the Celtics used a 53-27 run to lead by 26 points at halftime and then watched the Suns cut the deficit to 12 midway through the fourth quarter before prevailing, 123-108. It was the Celtics’ best win of the season, against one of the league’s elite teams.

Yet that wasn’t really the turning point. With Tatum out again, the Celtics needed a 50-point night from Jaylen Brown and a furious fourth-quarter rally to top the lowly Magic in overtime Jan. 2. Three days later, the Celtics dropped a home game to the below-.500 Spurs when a frantic comeback was ruined when Brown missed a tying layup at the buzzer.

This wasn’t a team that was starting to get it. The Phoenix win was impressive but it was followed by two disappointing performances. One night after the Spurs loss, the Celtics had yet another chance to change their growing label as one of the league’s biggest disappointments. And they played a nearly perfect 20 minutes, with Tatum back and splashing jumpers, for a 57-32 lead over the Knicks at Madison Square Garden.

The nationally televised audience saw the best of the Celtics, a team that was beating the Knicks with ball movement, strong defense, and sound point guard play.

Then it fell apart. The Celtics allowed a late second-quarter run, then yielded another 16-3 run late in the third. They lost the lead with two minutes left, only for Tatum to tie the score on a jumper with 2.2 seconds remaining.

If the Celtics had held on, maybe they would have felt better about their recent effort, maybe they wouldn’t be as self-deprecating because they did just enough to win. Except they didn’t. New York’s RJ Barrett banked in a winning 3-pointer at the buzzer.

The Celtics walked off the court as losers, having blown another large lead, with growing uncertainty over whether coach Ime Udoka was prepared for the responsibility of leading a franchise with high expectations. The players were shell-shocked. They had to face the media, completely embarrassed, a team with an 18-21 record yet enough talent to win the Eastern Conference but perhaps without the heart, fortitude, and desire to do so.

“I feel like the loss to the Knicks, that buzzer-beater, was kind of like the low point of the season and everybody kind of felt it,” Tatum said. “We all stayed the course, stayed together, and we all felt like it was just a matter of time. We just haven’t looked back.”

Something happened after that loss. Something dramatic.

The players and coaches don’t point to a signature moment such as an angry players-only meeting (a cliché) or Udoka tearing into his players and challenging their pride (that happened several times this season).

The change apparently was subtle, but it has resulted in a stirring turnaround, a 37-10 record since, including four consecutive playoff wins over the highly regarded Nets.

“Obviously there was frustration by losing another big lead,” Udoka said. “That’s where we put our foot down and said, ‘Enough is enough.’

“We had done well in games, did well enough to build these leads multiple times, and how can we hold on to those? I think the offense is a big part of what we talked about. We took a look at us as a coaching staff; players were tired of it as well.

“Obviously, I said some things in the media that got everybody’s attention.”

From Jan. 6 to the end of the regular season, the Celtics had the league’s second-best record, were third in field-goal percentage, fourth in 3-point percentage, fourth in rebounding, second in blocked shots, seventh in assists, and seventh in points per game.

They were No. 1 in point differential, outscoring opponents by an average of 13 points per game. The Grizzlies were second at 7.4.

Udoka said the coaching staff was almost forced to have one-on-one meetings with players because of COVID-19 restrictions, and he then implemented monthly sessions with all players and coaches. After the coaches offered players both praise and constructive criticism, they would leave the room for 15 minutes and allow the players to collaborate and then return for their feedback.

“We’ve had quite a few of those, and I think it encourages leadership, those guys being communicative,” Udoka said. “We show them where we’re at and the things we can improve on. What can we do better as coaches?”

Not only did Udoka ask for feedback from his players, he offered terse feedback of his own during this period. The players said they wanted to be coached harder after the Brad Stevens era, held more accountable for their actions, checked harder on mistakes, and expectations raised.

“At this point, I really just think Coach Udoka was really on us,” veteran center Al Horford said. “He was harping on us, November, December, into January, and the things we needed to be better and needed to do.

“Sometimes with groups, you get to a point where it’s either going to click and start moving forward or stay in that way and not much is going to happen. I feel like with our group, it started to click.”

Horford said with all the injuries and COVID-19 protocols, the Celtics didn’t gain a real understanding of what Udoka wanted until January. That comprehension along with a stretch of good health resulted in this run.

“We’re all teaching each other, learning from one another,” guard Marcus Smart said. “If somebody sees something if somebody doesn’t see, we’re allowing each other to critique and make those right decisions.

“The players-only meetings are so important because it’s one thing to have your coach show you this or show you that. It’s a difference when you have to be involved in it and you have to be on your toes, rather than having Coach talk and everybody sitting there quiet.”

Smart said the players decided that, win or lose, effort, mental lapses, and playing hero ball were no longer going to be issues.

“If we do that, nine times out of 10, we’re going to win games,” he said. “That was the biggest key for us, no matter the outcome, just play the game that we’ve always been playing.

“And Ime’s done a really good job of holding us all accountable, and we’ve done a great job of holding each other accountable. And that’s what happened and that’s what started that change.”