It’s not that coach Joe Mazzulla planned for this type of effort and performance, but it happened. The Mavericks were the more desperate team. Rookie center Dereck Lively hit his first career 3-pointer during an early Dallas run. Derrick White countered with an airball on a 3.

Nobody played well. The Celtics were punished on the boards. They couldn’t hit threes and they couldn’t hit twos. Now they return to Boston with a 3-1 edge in the series, but perhaps not the psychological edge because the Mavericks finally saw its bench produce while the duo of Kyrie Irving and Luka Doncic shredded a Boston defense that just wasn’t interested in guarding.

Both were able to drive to the basket with ease for layups and floaters. The Celtics perhaps believed the Mavericks were done, that they would lay down after trailing, 3-0, and on the brink of getting swept. Instead it was the Celtics who relented, reverting to their style of “playing with their food.”

The Celtics trailed by as many as 48 points in the third quarter, completely letting go of the rope and forcing Mazzulla to pull his starters with three-plus minutes left in the quarter. They needed to be benched. They were awful. Jayson Tatum missed layups or was stripped at the rim. Jaylen Brown looked two steps slow. Jrue Holiday missed a handful of layups.

White airballed another 3-pointer in the second half. And the Celtics faithful who traveled from Boston, snapped up the tickets on the secondary market and wanted to see history, actually witnessed one of the worst playoff losses in Celtics history. Only two previous Celtics teams lost by more points in the postseason.

What this loss does is create that slight doubt, that slight anxiety that maybe the Celtics have been figured out. The Mavericks played stifling defense in the fourth quarter of Game 3 but the Celtics made enough key buckets to hold on. On Friday, the Celtics couldn’t score in the paint and couldn’t hit even open threes.

Mazzulla was more complimentary of the Mavericks than critical of his own team, but he did think Dallas played harder. How in the world could the Celtics allow the Mavericks to play harder with so much on the line? Nobody had that answer.

“I mean, I don’t know, always in situations like this, it always goes back to us,” Mazzulla said. “But you have to give Dallas credit. They played well. They played really, really well. And that’s the reason why they’re in this, is because when they play like that. So I thought they just outplayed us. I don’t know. I thought Dallas played really [hard] — , because you have to give them credit. It’s not about us. It’s about our opponent and having respect for them.

“And you have to give them credit for how well they played, and that’s the most important thing. So it’s less about us and more about how they played. They played well. They played hard. We have to be more disciplined in how we defend and attack them.”

Adjustments will be necessary.

Mazzulla and his staff have two days to cool down Irving, make things tougher on Doncic, and limit the bench players who are now brimming with confidence.

The Celtics could have played one of their better games and still lost with the way Dallas performed, but they didn’t have a chance to find out because they didn’t offer their best.

Sweeps in the NBA Finals are extremely difficult. The last time an Eastern Conference team swept an NBA Finals series was 1989.

It’s not that the Celtics lost Game 4, it’s how they lost that is a concern. The Celtics still have the advantage. No team has come back from 0-3 and Boston is the better team, but they were so listless in Game 4 that they lost a considerable amount of momentum gained from such an impressive playoff run and winning the first three games of this series.

They’ll have to work hard to regain that and can’t just depend on being home to take this series.

“We learn from it. We take it. We don’t dismiss it,” guard Jaylen Brown said. “We’re going to learn from it. We’re going to see how and why, exactly where the game was won and lost. And then we take those experiences and then we come out and we play like our life depends on it. Because it does.”

The Larry O’Brien Trophy will be shipped to Boston for Game 5, and the Celtics have the precious opportunity to clinch in front of a title-hungry city. They’ll have to play with the ultimate passion, no arrogance and with all the sense of urgency.

Gary Washburn is a Globe columnist. He can be reached at gary.washburn@globe.com. Follow him @GwashburnGlobe.