DALLAS — There are losses that are tough to swallow. And then there are losses when you give up 51 points in a single quarter.

The fortunes of the Lakers could be read plainly on LeBron James’ face in their Christmas Day matchup with Dallas. As they cruised out to an early double-digit lead, he grinned, toying with his mouthguard, chattering to Mavericks coach Jason Kidd and showing mock disdain for a Dallas defense that couldn’t stay between him and the rim.

Then by the end of the third quarter — a period in which the Mavericks outscored their guests 51-21 — cameras caught James on the bench slumped, his hand cradling his face with his eyes closed.

In his defense, the collapse was hard to watch.

A 124-115 defeat to Luka Doncic and Dallas was the fourth straight for the Lakers (13-20), another downward step in another challenging season.

“We talked about it before the game: just being able to initiate, sustain and finish with energy, effort and competitiveness,” Coach Darvin Ham said. “And we left it in the locker room at halftime.”

James was often brilliant when he was on the court, scoring 38 points on 13 for 23 shooting and going 12 for 12 at the line. But without Anthony Davis as a companion star, the rest of the Lakers’ roster continues to collapse under the strain — the holidays being no exception.

James has now scored at least 30 points in seven straight games, and the Lakers are just 3-4 in those contests, including three straight losses. The team is 2-4 since Davis was injured earlier this month.

The Lakers have squeezed their way out of many a tight spot already this season, since starting 0-5, then 2-10. But while in one light, that might make a group feel battle-hardened, James sounded weary of constantly digging his way out of holes not even halfway through the season.

“I look at it the other way too,” he said, “like, how many times are you going to try to dig yourselves out until it’s too much dirt on you?”

Doncic, who many consider James’ successor as a great playmaker who is also an elite scorer, finished with 32 points, 9 rebounds and 9 assists.

Before halftime, one would be hard-pressed to guess which team has been struggling of late. A vintage effort from James had the Lakers up 23-9 in his first shift, crossing through the defense with bullying drives and deft, spinning touch.

A risky defensive strategy of doubling Doncic was working early on: At halftime, the Lakers led 54-43 and the Mavericks were shooting under 40% and 5 for 22 from three.

The success was short-lived.

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The Lakers quickly were overwhelmed once the Mavericks’ shooting picked up in the third quarter, arriving late in their rotations to shot after shot. Dallas responded by hitting 9 of their 13 looks from deep in the fatal frame. The Lakers also couldn’t guard Doncic one-on-one, either — few teams can — and notably Russell Westbrook was burned on three straight possessions guarding the 23-year-old MVP candidate.

Throughout the season, the Lakers have been haunted by poor third-quarter efforts, a trend that’s been exacerbated by Davis’ absence, giving up at least 124 points in each of the last four games.

“First half has been great. First half last game was great. First half game before was great,” Patrick Beverley said. “You know, something about that second half – I don’t know if it’s mental, I don’t know if it’s effort, if it’s– you know, (expletive) what it is. But I’m not gonna sit up here and make a ton of excuses, whatever it is, we need to get better at it defensively.”

A huge difference was that Doncic had offensive help: Christian Wood had 30 points, 8 rebounds and 7 assists, while Tim Hardaway Jr. had 26 points. Coming on the heels of a 50-point game by Doncic, spreading the wealth helped the Mavericks.

James had more points than the rest of his starting lineup (32 points). Westbrook added 17 points and Reaves had 16 points, both off the bench — but Westbrook’s minus-30 rating to James plus-2 was an indicator of how poorly the Lakers played with James on the bench. Ham reached for different lineups with erratic results, even playing an all-guard lineup with Westbrook at center.

“You throw everything up against the wall and see what sticks,” Ham said. “It’s one of those type of situations.”

The Lakers hoped that they might have fouled Doncic out with just under four minutes to go in the fourth, but Kidd successfully challenged the call, changing the charge into a block on Austin Reaves. Doncic wound up icing the game with free throws and a late 3-pointer over Dennis Schröder.

James played in a record-setting 17th Christmas Day game. Though the 37-year-old isn’t a fan of early tipoffs like the 1:40 p.m. CT start in Dallas, playing on Christmas against a coaching staff he knows well gave him “a little more mojo,” he admitted.

But ultimately, the Lakers are skating on razor-thin margins. While the way the game slipped away from them was disappointing, James didn’t think the result was surprising.

“We came out with a gameplan, it worked the first half,” he said. “We could have made some adjustments, but it’s not the reason why we lost the game. But reality is, without AD, we lose a lot of length which we don’t have already. So we have to make up in ways that, without AD, is very difficult, very challenging. … So, you don’t have to be a rocket scientist to figure it out.”