Never imagined the defending NBA champion Boston Celtics would exit the playoffs one year later in need of a Mercy Rule.

But that’s how it was for the Celtics in do-or-die Game 6 against the New York Knicks at Madison Square Garden.

Someone needed to stop the fight, only it carried out to the bitter end.

The Celtics were beaten to a pulp, whipped to the point of embarrassment. They hung in for a quarter, but the rout was on before halftime.

The end couldn’t come soon enough. It got so bad the C’s trailed by 41 with 1:49 to play in the third quarter en route to losing 119-81 in the series clincher to the Knicks.

When the drubbing was over, perhaps it was Jaylen Brown, who fouled out in the third quarter, who summed up the evening best: “Losing to the Knicks feels like death.”

At the outset, the Celtics stated goal was to repeat, especially given they were returning the same team from last year.

For much of the year, going back-to-back didn’t seem to be out of the realm, even if that was a rare occurrence in the NBA recent times.

They seemed driven to make it happen. But the obstacles thrown in their path were too great to overcome.

The absence of Jayson Tatum, who ruptured his Achilles tendon late in the Game 4 loss, was a factor. So was Kristaps Porzingis’ mysterious illness.

But the way the Celtics played in the postseason as a whole, didn’t have much resemblance to what we witnessed last year.

They lacked urgency. They didn’t play with the same edge or purpose. They blew a pair of 20-point leads. And another 14-point lead against the Knicks.

And in the final act, they pulled a complete no-show in do-or-die Game 6. The lasting image isn’t pretty.

The Knicks wanted it more. They were hungrier and played with more desperation, coming back from those double-digit deficits at the TD Garden in the first two games to jump out to an improbable lead in the series.

The Celtics showed some mettle in Game 3, and especially Game 5, the first without Tatum. But in the final analysis, they were woeful in crunch time against a team they had beaten four times – with no losses – during the regular season.

Joe Mazzulla’s team fell in a hail of missed threes and turnovers. They were dominated on the boards, crushed by second-chance points allowed. They had no semblance of defense or offense during the key junctures of games.

It was pretty much the same formula for defeat when Tatum played. Even if he was healthy, the Celtics were going to be goners.

That’s how it looked.

The team, as a whole, seemed a step behind for much of the series.

“Obviously, you win a championship, and you’ve got that target on your back from Day 1,” Derrick White said. “There’s ups and downs through every season. This part sucks. Just wasn’t able to complete the goal we set out to get.”

The Celtics aren’t alone. Plenty of other teams that looked like juggernauts before them weren’t able to go back to back.

Many, like the Celtics, didn’t make it past the second round the following season. Denver, Toronto, Milwaukee and the LA Lakers are recent examples.

“At the end of the day we set out a goal, and didn’t achieve that goal,” Mazzulla said of pulling off back-to-back titles. “But, it shouldn’t take away from the mindset and the effort the players put in. We have a responsibility and ownership. We didn’t do it. But the approach and process to it, you can’t ask any more from the guys.”

With new ownership, a payroll up around $228 million with a $280 million luxury tax bill projected, the players were well aware this was the last kick at the can with this group.

There will undoubtedly be changes, starting with the fact Tatum might not be around for much, if not all, of next season coming back from that type of injury.

Al Horford’s contract is up. At times against the Knicks, he looked his age (38) especially having to play more minutes with Porzingis ailing.

Will he want to come back in a non-contending situation with Tatum out? Will the Celtics want him back?

Jrue Holiday (34), who was 1-for-8 in the clincher, didn’t look much like the player he was last year. He fought through injuries all year.

Both he and White, who were a combined 2-for-10 from three-point land, went cold at the wrong time. Holiday is on the books for $32.4 million next season, which is third behind Tatum and Brown.

Will the salary-chopping start with him?

Beyond personnel, Mazzulla and the staff also have to see if their play style – setting records for three-pointers attempted – still makes sense.

A string of cold nights in the playoffs – they went 12-for-41 from three Friday night – and you’re done.

Needless to say, there’s a lot of decisions to make, and work to do to try and keep the team competitive during Tatum’s expected absence.

“Losing stings especially finishing the season like this,” said Brown, who battled through a knee injury. much of the year. “It just wasn’t our year.”

They went from a Duck Boat parade in downtown Boston, to a gleeful MSG crowd singing “Na Na Na Na, Hey, Hey, Hey, … Goodbye” during the final minute.

Said Porzingis: “It was a tough way to end.”

In more ways than one.