NEW YORK -- After the Boston Celtics blew their latest lead of the season in a heartbreaking 108-105 defeat at the hands of the New York Knicks at Madison Square Garden, coach Ime Udoka repeatedly called out his team for a lack of mental toughness.

"I think it's a lack of mental toughness to fight through those adverse times," Udoka said after Knicks forward RJ Barrett's banked-in 3-pointer at the buzzer completed a 25-point Knicks comeback to stun the Celtics. "To your point, it's across the board. It's a turnover here, a bad shot here, a missed defensive assignment here, and several missed rebounds tonight.

"So, it's a lot of different things. And then, like I said, a calming presence to slow it down and get us what we want is really what you need at that point. And sometimes we all get caught up in it."

Boston has now blown four leads of at least 19 points this season -- two more than any other team in the NBA -- and has been one of the NBA's worst fourth-quarter teams this season. The Celtics rank 26th in offensive efficiency and 25th in defensive efficiency in fourth quarters, and are 29th in net rating, being outscored by 8.7 points per 100 possessions.

And, after Thursday's loss to the Knicks, they find themselves sitting in a tie with Atlanta for 11th place in the Eastern Conference -- outside of not just the playoffs but the play-in tournament spots, as well.

Many of the losses that have put the Celtics in that predicament have come in a similar fashion to the way they lost Thursday -- fourth-quarter collapses and late-game breakdowns. Udoka said in order for that to change, his team's approach has to change in those moments.

"It's guys getting rattled when it's not the end of the world," Udoka said. "You still have a 12-point, comfortable lead, and you gotta end that run.

"We have to understand time and score, and we need a solid shot and not just get caught up in the game. You have to play the game different in the first quarter than the fourth quarter. Different when the team is going on the run, and when you're getting stops and getting out running.

"It's not open gym. You can't just play the same way the whole game. So we have to have better understanding of that, and get the quality of shot that we want to stop a run and not just get caught up in it and four, five guys make the wrong play and it starts to snowball."

The latter scenario has happened too often to Boston (18-21) this season, including in Thursday night's loss. After controlling the game for the entire first half, the Celtics allowed the Knicks to storm back into the game with two separate lengthy runs in the third quarter, before New York eventually came all the way back in the fourth.

"We just have to regroup," said Celtics star Jayson Tatum, who had 36 points, including a jumper with 1.5 seconds left that, at least briefly, tied the game for Boston. "Obviously losses, this stretch kind of is tough, losing leads and just not finishing out in the fourth quarter.

"I think we just have to look at the big picture and the grand scheme of things and look back to those years when we were going to the conference finals and it makes you really appreciate those moments. Because it's hard. It's not easy.

"I think early on, probably my rookie year, I thought that was just normal. Winning all those games, winning games in the playoffs, probably taking it for granted a little bit. But to enjoy those moments, stuff like this happens and I think it makes you appreciate times like that even more, just knowing how hard it is to win in this league."

Boston is certainly learning that the hard way -- and over an extended period of time. After the Celtics made it to the Eastern Conference finals three times in four seasons, Boston was exactly .500 during the regular season last year, and is now guaranteed to be under .500 when it reaches the halfway mark of the season next week.

Throughout this season, Udoka -- in a stark departure from the way his predecessor, and now Celtics president of basketball operations, Brad Stevens, would operate with the media -- has gone after his team for its play on the court in postgame media sessions.

But Thursday's session was arguably his toughest criticism yet, when he pointedly said his team needs to be smarter and tougher in late-and-close situations.

"It's been the same result and it's some kind of mental toughness there, where something goes a little bad and we all start to drop our heads," Udoka said.

The Celtics will now head back to Boston to face the Knicks again Saturday night, where they'll try to rectify their troubles and get their season back on track.

"We get rattled a lot, especially when we're facing adversity," Celtics center Robert Williams said. "We just have to find it in ourselves, the grit and the fight, to just come together when something's not going our way."