Porzingis scored only four points on 1-of-11 shooting in 20 minutes in the loss. He sat out the final nine minutes, four seconds after picking up his fifth foul, a decision Carlisle attributed to Porzingis' "tough night."
"There's nights like this," Carlisle said. "We're just going to flush it and move forward."
This outing was the low point of what Porzingis has acknowledged has been a frustrating start to his Mavs tenure. He has struggled to find a rhythm in the offense after missing 20 months while recovering from a torn ACL in his left knee suffered in his final game for the New York Knicks, who traded the 2018 All-Star to the Mavs last season.
Although Porzingis didn't want to watch from the bench as the Celtics pulled away in the final minutes, he couldn't argue that he merited clutch minutes on Monday.
"Of course I want to be out there, but can't blame him," Porzingis said. "I wasn't having a great game. I'm all-in for whatever's best for the team. If the coach thinks he'd rather have me out and have someone else in that's having a better game, let's do it if we can win a basketball game. That's the most important thing, but going forward, I want to make sure I'm out there."
Porzingis, whom the Mavs signed to a five-year, $158 million maximum contract in July, is averaging 18.3 points on 40.1% shooting from the floor in nine games for the Mavs. There have been glimpses of the 7-foot-3 Latvian's immense potential, such as his 32-point performance against the Portland Trail Blazers and his 28-point performance against the Knicks, but he has been held to 10 or fewer points in three of his past six games. Porzingis admits that he has a tendency to press when he isn't as involved in the offense as he'd like.
"Sometimes I'm not going to get the ball as much as I'd like to, and that leads to me maybe forcing some things and so on," Porzingis said. "It's a work in progress. I know Coach is trying to do the best job he can to get me the ball in the offense and utilize my skills, and I also got to do a better job of making sure I'm effective in those situations."
Porzingis has especially struggled in post-up situations, which usually come when he's defended by smaller players after switches. According to NBA.com stats, Porzingis was averaging only 0.64 points per possession on post-ups entering Monday, which put him in the 12th percentile in the league. Before being benched, Porzingis missed a pair of turnaround jumpers while defended by Boston guard Marcus Smart, who drew an offensive foul on another post-up on Porzingis' final play of the night.
"They did a good job switching and getting underneath me," Porzingis said. "I had no legs on my shot. It was just flat. I rushed into a couple of them. I have to take my time when I get those switches. Get down low, get good position, and just turn around and shoot over guys."
The Mavs anticipated that Porzingis would need time to work his way back into form after such a long layoff. They're taking a patient approach with a player the franchise is banking on being a long-term foundation piece.
"This happens to great players. They have bad nights," said Mavs star Luka Doncic, who had 34 points and nine assists in the loss. "I think the next game he's going to have at least 30 points. He keeps working. He just came back from injury. These things are normal. We all believe in him."
Porzingis, who makes his highly anticipated return to Madison Square Garden on Thursday, has accepted that it will be a challenging progress to regain his groove while adapting to a new system and teammates.
"It's not going to happen overnight," he said. "We're 6-4 with me playing below the level that I'm used to. Once I find my game and be effective out there, we can be doing much more damage."