DALLAS (AP) — Jayson Tatum had his step-back 3-pointer swatted away, then ended up in a heap on the floor and was called for a loose-ball foul.

It was that kind of night for Tatum and the Boston Celtics. They got knocked down hard in Game 4 of the NBA Finals when they had a chance to complete a sweep of the Dallas Mavericks.

Instead of wrapping up an unprecedented 18th NBA title, the Celtics suffered their worst loss ever in the NBA Finals — and one of the worst in league history — when they fell 122-84 on Friday night.

“It happened, we can't change it. We had a bad night,” Tatum said. “We always say you lose by two or you lose by 30, they all count the same. ... We’re not making any excuses. We need to be better, and we will.”

The Celtics’ 10-game postseason winning streak, a franchise record, ended after they lost on the road for the first time in these playoffs. They had been 7-0, including a Game 3 win in Dallas after overcoming a 13-point deficit midway through the first quarter.

Now the Celtics will get a chance to clinch it at home. Game 5 is in Boston on Monday night.

Dallas already had a 26-point lead at halftime, and any thoughts of a rally by the Celtics were pretty much done less than two minutes into the second half when Tatum had his shot blocked by Daniel Gafford, with the Boston forward reaching out as he fell and getting whistled for the foul.

With the outcome already all but certain, coach Joe Mazzulla emptied his bench with 3:18 left in the third quarter. It was then a 36-point margin, and grew to as much as 48.

“I thought the guys came out with the right intentions. I just didn’t think it went our way, and I thought Dallas outplayed us. They just played harder,” Mazzulla said. “You have to prepare to put yourself in the best possible position, but it doesn’t guarantee anything. At the end of the day, we have to just maintain our process and get ready for Game 5.”

Tatum, who had 15 points and five rebounds, was on the bench with Jaylen Brown and the rest of the starters for the remainder of the night. And 7-foot-2 center Kristaps Porzingis never even removed his warmups after being declared available before the game.

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

There have been only two more lopsided games this late in the season: Boston's 131-92 win over the Lakers in Game 6 to wrap up the Celtics' last championship in 2008; and Chicago's record 42-point win over Utah in Game 3 in 1998.

The Celtics had their lowest-scoring half all season — regular season and playoffs combined — when trailing 61-35 at halftime. They had missed 26 shots and had only two offensive rebounds.

When it was all over, they had been outrebounded 52-31 and outscored in the paint 60-26.

“We can say all these things about us. They played much better than us,” said Al Horford, the 38-year-old center seeking his first NBA title. “They clearly outplayed us, and that’s tough to take. But that’s the reality.”

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