The Celtics could have made excuses on Monday night if they wanted to.

The schedule was not in their favor. They were playing the second night of a back-to-back on the road against a Mavericks team that had five days of rest after their game against the Warriors last week was postponed.

The C’s were missing a key starter, without Kristaps Porzingis.

But these Celtics refuse to give into narratives or excuse-making. As the dog days of January threaten to put teams in the lull, the Celtics are viewing this stretch leading into the All-Star break as an opportunity to strengthen their focus and build necessary habits for their postseason run.

Jaylen Brown has been the leading voice in that movement, and he backed it up again on Monday. He scored 35 points, including some big shots down the stretch. Jayson Tatum scored 38 points, as the Celtics stars got back to their scoring ways a night after being limited by the Rockets. The duo also led the way defensively in the closing moments as they held off a late surge by Luka Doncic and the Mavs to grab a 119-110 victory.

“This is the tough part of the season, and like mindset and mentality is going to shine through,” Brown told NBC Sports Boston’s Abby Chin. “This was a good example of that tonight, on a back-to-back. They came out starting out hot and we were just able to roll on. Down a guy, KP our big fella wasn’t playing, but we found a way to get a win anyway.”

Doncic posted a triple-double with 33 points, 18 rebounds and 13 assists, but shot 12-for-30 in a highly inefficient night. The Mavs star made a late push for a comeback, but even on the second night of a back-to-back against the well-rested Mavs, the C’s had plenty in the tank to ward off a late collapse.

The Celtics led by as many as 16 and had a 14-point advantage midway through the fourth quarter as Tatum, Brown and Jrue Holiday (17 points, five 3-pointers) built a big lead by torching Dallas’ mediocre defense with hot shooting nights, but even though Doncic had an unusually rough shooting night, he helped the Mavs hang around late. He led an 8-1 run capped by a step-back 3-pointer over Derrick White to cut Boston’s lead to seven with five minutes to go, and it looked like the Mavs star – who has torched the C’s in two previous meetings – had some more magic for them.

But the C’s, facing some sudden stress, answered the right way. Brown was clutch down the stretch. He responded by hitting a turnaround jumper, and after Doncic got to the line and sank two free throws, Brown answered with a four-point play after drilling a 3-pointer and crashing to the floor in front of the Mavs bench.

On the next possession, Brown blocked Maxi Kleber’s layup and the Celtics made enough plays in the closing minutes to seal the victory. Holiday made a short jumper to put the C’s up 111-100 with 3:11 to go, and after Kyrie Irving – who Holiday shut down for most of the night – hit a 3-pointer with 2:25 to go to slice Boston’s lead to eight, Brown made another jumper to put the game away.

Other takeaways from the win:

– Brown seemed to take this game personally as he matched up with Doncic on both ends. Brown clearly outplayed Doncic, who was visibly frustrated for most of the night. On one stretch in the second quarter, Brown scored seven consecutive points, which included one play in which he dropped Doncic on a step-back jumper and then converted a three-point play on him. He was also the primary reason Doncic had a poor shooting night as he bothered him all night.

“I feel like I’m one of the better defenders in the league, if not the better defender in the league so it’s a challenge that I think should be bestowed on me,” Brown told NBC Sports Boston. “Whatever the night, my body’s in a different shape, I’m guarding the best players night to night.”

– Tatum was mired in a four-game shooting slump – 35.8 percent from the field and 21.4 percent from deep – that included a 4-for-17 effort in Sunday’s win over the Rockets, but quickly shook it off with an aggressive first quarter. He scored 13 points via a 4-for-6 start in the opening period – his most in a first quarter this season – as he got to the free throw line five times and didn’t take a 3-pointer. His strong three-point play at the end of the quarter cut Boston’s deficit to just two despite a slow start offensively.

Tatum, who had just 18 points in Sunday’s win, matched that total on a pull-up jumper with 8:02 remaining in the second quarter.

– Monday marked the Celtics’ first meeting against old friend Grant Williams, who spent his first four seasons in Boston before being sent to Dallas in a sign-and-trade this summer. Williams, who has struggled in his transition, was a non-factor off the bench with two points and three rebounds in 18 minutes. On the final play of the first half, he attempted a buzzer-beating shot but Tatum blocked him and had some words for his friend and former teammate on his way to the locker room.