The Celtics had an opportunity to clinch their 18th NBA championship Friday night.

Instead, they were treated to a Texas-sized blowout at the hands of the desperate Dallas Mavericks.

The Luka Doncic-and Kyrie Irving-led Mavs routed the Celtics 122-84 to prevent an NBA Finals sweep and send the series back to Boston for Game 5 on Monday.

“That was a butt-whupping,” Celtics backup Sam Hauser said. “That’s it. They just beat us. Not much to it, honestly.”

It was the third-largest point differential ever in a Finals game and easily the worst game of the season for the powerhouse C’s, who hadn’t lost since May 9.

What went wrong? Celtics stars Jayson Tatum and Jaylen Brown offered their opinions after the game.

“I think this is the most stagnant that we’ve been this series and the worst job of owning our space on the offensive end and doing what we wanted to do instead of what they were forcing us to do,” Tatum said. “We did a great job of that the first three games, and this one, we didn’t.”

“Give credit to Dallas,” Brown added, repeating a popular refrain from Boston’s postgame news conferences. “I think they played hard physically, and then they dominated the glass. I think that was the key that put a lot of pressure on us. And then we didn’t make shots tonight, and I think that’s where we allowed them to play free, once we get them multiple opportunities to make shots. And then also we didn’t hit shots on the other end.”

Dallas outrebounded Boston 52-31, including 13-4 on the offensive glass. The Mavs shot 50.5% from the floor and 40.5% from 3-point range — compared to 36.3% and 34.1% for the Celtics, respectively — and outscored the Eastern Conference champs 60-26 in the paint.

Two days after combining for 61 points in a thrilling Game 3 win, Tatum and Brown went a combined 7-for-22 and 2-for-9 from three. No Celtics player scored more than 15 points.

The Mavericks led by as many as 48, and in a display of surrender rarely seen in the NBA Finals, Celtics coach Joe Mazzulla pulled his starters before the end of the third quarter.

“Close-out games are hard,” Brown said. “Close-out games are tough. They always have been like that, and you’ve got to have extreme focus. You’ve got to come out and meet their intensity to finish things out. We didn’t do that tonight. Give credit to Dallas. I thought they played extremely well. … We’ve got to be better and we’ve got to have some of our guys step up. That’s what it takes.”

The Celtics still are in command of the best-of-seven series, and their prior championship runs followed a similar pattern. In 2008, 1986 and 1984, Boston lost a potential close-out game on the road before winning the title at home. No team in NBA history has lost a playoff series after winning the first three games.

Tatum expects TD Garden for Game 5 to be “as loud as it’s ever been in my seven years of being a Celtic.”

“(We’re) excited to go back home,” he said. “Celebrate Father’s Day on Sunday and compete for a championship on Monday. It should be a lot of fun.”

Title perspective

Veteran Jrue Holiday offered a simple analysis of what happened in Game 4, while dismissing the idea the Celtics softened up pregame and believed their own hype.

“I think winning is hard. I think winning at any game is hard. But winning Game 4 of the NBA Finals is pretty damn hard. So no, I think they came out desperate and I think they punched us in the mouth,” Holiday said. “And we couldn’t kind of recover the way we wanted to.”

Holiday, who coughed up five turnovers and was a team-worst minus-38 in the loss, is the only player on Boston’s roster with a ring. However, he didn’t have much to offer when asked what the Celtics must do to clinch Monday night.

“I don’t know. I don’t know, even though I’ve done it,” he said. “You’ve got to do it together as a team. Everybody has to be clicking and being, again, the more desperate team. We’ve just got to go back home. I think locking into tendencies, locking into the game plan, and I think we’ll be.”

Holiday did say the Celtics must be the more desperate team, a common refrain for players and coach Joe Mazzulla this postseason. Mazzulla said post-game Dallas’ effort was “a lot better,” and his players agreed.

“Yeah, I felt like we came out determined,” center Al Horford said. “They were the better team. They were the team that was playing harder. He’s right.”

Off the rim

Dallas coach Jason Kidd had a different view of the Celtics’ preparedness. He said Boston played like a team that believed it had already secured the championship. “We don’t have to complicate this,” Kidd said. “This isn’t surgery. Our group was ready to go. They were ready to celebrate.” … The win snapped a 13-game personal losing streak for Irving against the Celtics. He’s now seeking his first win at TD Garden since May 2021. “When we go to Boston, there’s going to be a bunch of (fans) yelling a whole bunch of crazy stuff still,” Irving said. “But I think we’ve been able to grow and face kind of this adversity head on.”