Back when the Washington Wizards and Boston Celtics had a robust rivalry, it couldn’t be separated by a thin black curtain.

Jae Crowder crossed that line in January, pushing aside the curtain that separates the locker rooms at TD Garden after the Boston forward and Washington star John Wall got into it following a Celtics win. The incident grabbed the league’s attention — Crowder was slapped with a $25,000 fine, and Wall was docked $15,000 — and launched the Wizards-Celtics rivalry from a handful of regular season dust-ups to an epic, must-watch conflict, blossoming into seven games of rage, fire and fun in May’s Eastern Conference semifinals.

“We wanted to kick those guys’ [butt] every time we saw them,” said Crowder, who also had gotten into it with then-Wizards coach Randy Wittman a year earlier. “We hated those guys. . . . When we got matched up against them in the playoffs, we were licking our chops. We wanted those guys.

“It was a great series for the viewers. It went seven games and it ended in a great fashion. But it was special. That whole series was special.”

Yes, it was special. Was.

As the Wizards stayed the course, an offseason remodeling stripped the Celtics of five of their seven most-used players from last season, including Crowder, and perhaps stripped one of the league’s most entertaining rivalries of much of its luster.

“I think it was a lot of no love between two teams. We didn’t like each other. Maybe we respect each other a lot but we didn’t like each other,” Wall said. “That’s the reason why we have almost damn near every game [between the two teams] on national TV, because they thought it would be the same rivalry, but they’re a different team.”

The NBA and its television partners are hoping the rivalry has some life left, placing Wizards at Celtics (5:30 p.m. Eastern time, ABC) in the center slot of its much-hyped five-game Christmas Day slate, even if some of the principal agitators are in different uniforms or speak of the hate in past tense.

“I don’t think it’s as deep as it was,” Wizards forward Markieff Morris said. “They got a whole new team. You just can’t have rivalries when organizations change.

“I don’t think it’s going to be the same. I honestly don’t.”

But when it was hot, it was scorching. These guys can’t even agree on what happened that night in Boston.

It started with Crowder and Wall invading each other’s personal space after the buzzer, close enough that Crowder pointed his index finger at the tip of Wall’s nose. Wall slapped Crowder’s hand. The two were separated.

“You just sensed something was happening,” said Phil Chenier, the Bullets legend who worked the broadcast that evening and noticed the commotion.

As the teams left the floor toward opposite exits, players continued the war of words. Only about 15 feet separate the Celtics’ home and visitors’ locker room, the curtain dividing them. In Crowder’s six years in the NBA, he had never tried anything like this before — but then again, no team had made his blood boil quite like these Wizards.

Security in the hallway requested backup. Police from the court sprinted to the locker rooms. Once Crowder pushed aside the curtain, Wizards players stood at the door, ready.

“You can’t let guys cross over to our locker room,” Wizards Coach Scott Brooks said. “What do you expect? What do you want us to do? Go run and hide?”

Crowder said he attempted to get into the Wizards’ locker room. Morris disagreed.

“He didn’t try to get into no damn locker room. He was on the other side of the curtain,” Morris said. “If they wanted to fight, they could’ve fought right there. That’s just basketball talk.”

Those squads played nine more times, and each game stoked the flames hotter.

The Wizards showed up to their home arena wearing all black for their next regular season meeting with Boston, which Wall had said would be a Celtic “funeral,” and the normally calm Bradley Beal was seeing red. The guard scored 31 points, none louder than an and-one against Boston’s Marcus Smart, which left Beal clenching his fists and screaming. Wall head-butted Beal backward.

“That’s probably the most expression I’ve showed,” Beal said. “We took it serious. We both took it to heart. It was a passionate game.”

Even Morris — who typically wears a poker face and has a low baritone that barely registers at audible levels — broke character. In the first game of the conference semifinals, Morris attempted a jumper and landed on the foot of Celtics center Al Horford. The fall badly sprained his left ankle, which still bothers him. Morris missed the rest of that game but returned to the starting lineup with the unshakable feeling that Horford had intentionally stepped into his landing space.

Before the tip-off for Game 2, Morris walked over to Horford and calmly said: “Don’t let me find out you tried to do that on purpose. We’re going to have a problem.”

Horford responded, “I don’t play like that,” according to Morris. But it didn’t matter. Less than two minutes into the game, the burly Wizards forward grabbed Horford by the waist and shoved him into the baseline seats, into the lap of Wizards majority owner Ted Leonsis.

“I owed him that,” Morris said recently. “He sprained my ankle. I still feel like he did that on purpose.”

Horford is still in Boston, one of the few remaining pieces from last year’s team. Only three other players — Smart, Jaylen Brown and Terry Rozier — from the 2016-17 15-man roster are back.

“I went to the Celtics’ first day of training camp and thought, ‘Well, this team is entirely different than it was a season ago,’ ” said Doris Burke, who will serve as ABC’s analyst for the Christmas Day game alongside play-by-play announcer Dave Pasch. “I almost laughed out loud at how different they are.”

Despite finishing in first place in the East and making the conference finals last season, the Celtics underwent a massive roster renovation.

In June, the team traded its No. 1 overall spot in the NBA draft to the Philadelphia 76ers for the third pick, selecting Duke’s Jayson Tatum. To free up money for a max-contract free agent, the team allowed Amir Johnson and Game 7 hero Kelly Olynyk to walk in free agency then traded Avery Bradley to Detroit in exchange for Marcus Morris, twin brother of Markieff. Boston landed highly sought-after free agent Gordon Hayward in July, then shocked the basketball world a month later when it acquired another all-star: Kyrie Irving. The guard swapped LeBron James’s shadow for Boston’s spotlight, replacing beloved guard Isaiah Thomas, who finished fifth in MVP voting. Crowder accompanied Thomas to the Cleveland Cavaliers.

“It’s not really [about] looking to make changes or desperate to make changes or feeling that changes need to made. I would say this offseason, we were opportunistic,” said Danny Ainge, the Celtics’ general manager and president of basketball operations. “I loved our team last year. It was as much fun as I’ve had doing this job in 15 years. I loved the personalities and I loved their grit and I had a blast being with them the last few years and I love this team equally that we have now.”

Despite all the turnover and losing Hayward to a gruesome fractured ankle in the season opener, the revamped Celtics look even better — winning 16 straight early in the season and soaring to the top of the East.

“Boston has built its roster with players who win in today’s NBA,” said Burke, noting the versatility throughout the roster, which optimizes switching on defense. “What they have in Kyrie Irving is truly extraordinary.”

The Wizards went the exact opposite direction last summer, doubling down on consistency. The team re-signed Otto Porter Jr. and extended Wall’s contract through the 2022-23 season as part of a strategy to keep the core together and add incrementally around Wall, Beal and Porter.

“It’s important to have continuity when your guys are good guys and will continue to improve,” Brooks said. “Our nucleus is young.”

Even with their core players back, the Wizards have floundered this season. The chemistry hasn’t quite kicked in. The core has already missed more games to injury than all of last season — Morris sat the first seven, Wall was inactive for 11 and Porter has been sidelined in two games leading up to the Christmas matchup. The result: a middling record with inexcusable losses throughout.

Even so, Ainge praised the Wizards’ path.

“I like what they’re doing,” Ainge said. “They’re doing some really good things and it doesn’t really matter how you get there and I think everybody ideally prefers continuity. Continuity is a good thing and I’m not sure there’s anybody that disagrees with that.

“They certainly have our respect as being a much, much better team than their record is right now,” he continued, “and we know that Christmas Day game will be a very difficult one for us to win.”

Morris won’t have revenge on his mind this Christmas, spending time with his baby daughter, twin brother and their mother. Too much has changed, he said.

“They don’t have the same type of guys,” Morris said. “You had Isaiah who’s barking at you. Jae Crowder who’s barking at you and would make the other guys bark. They don’t have those type of guys anymore. They just have guys that just come in and play the game.”

Well, Smart remains on the Celtics, and the pesky guard is sure to remember the bad blood.

“Oh yeah, Marcus is going to keep it going,” said Crowder, whose Cavaliers also will play on Christmas Day, in an NBA Finals rematch with the Golden State Warriors (3 p.m. Eastern time, ABC). “Marcus don’t like these guys either. It’ll be fun.”

Ainge said he believes the rivalry can survive Boston’s roster upheaval because the goal remains the same for both teams: supplant Cleveland atop the East.

“It was a fun rivalry. The players were into it. It was for a chance to get to the Eastern Conference finals,” Ainge said. “I think that’s why it’s developed into a rivalry over the last couple of years with Washington because we’re sort of trying to get to the same place: respect among the league and in the Eastern Conference.”

Both sides can agree on one thing: Last year was special.

“I would say [the rivalry’s] still there,” Wall said. “I wouldn’t say it’s strong because key guys are gone but some of those guys still remember what was going on last year and how it was going. I think we still remember.

“Even though those guys aren’t there, we still feel like Boston is the team that knocked us out of the Eastern semis, so we want to get back at those guys,” Wall continued, “but we’ve got to do it better by winning some damn games.”

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