It was supposed to be a coronation, but instead was a stunning, wasted opportunity for Celtics

Jayson Tatum reacted with disbelief after a call late in the fourth quarter was overturned, and an offensive foul had just been called on Jaylen Brown, effectively ending any chance Boston had to win.Jim Davis/Globe Staff

Are you kidding me?

We bought plane tickets for San Francisco. We waxed poetic about the “Dream Finals” between the Boston Celtics and the Golden State Warriors. Even veteran Warrior Draymond Green said the Warriors “are going to play Boston” in the league’s showcase event.

Most everybody dismissed the poor, pitiful Miami Heat as mere cannon fodder for the Celts after Boston dominated the Heat in Games 4 and 5 of their Eastern Conference final. It was at the point where we were almost feeling sorry for the banged-up, worn-down Heat.

Celtic Nation filled the New Garden on a festive Friday ready to see Jayson Tatum, Jaylen Brown and Marcus Smart finally ascend to the ultimate series. It was supposed to be a coronation.

Instead, it was a stunning loss for Boston. Playing in the image of their front office leader (Pat Riley), the mentally tough, ever-proud Heat quieted the Garden, led most of the night, and beat the overconfident Celtics, 111-103, to force at Game 7 Sunday night at FTX Arena in Miami. Two missed free throws by Brown (2 points in the second half), a turnover by Tatum, and a missed jumper by Al Horford (3 points) crushed the Celtics in the final 2:18. Jimmy Butler led the visitors with 47 points and played 46 of the 48 minutes.

“Jeez,’’ mused former Celtic great Cedric Maxwell. “And I was all ready to hand out the Larry Bird trophy to the series MVP.’’

There’s still a chance the Celtics will beat the Heat Sunday and fly west to start the Finals against the Dubs at the Chase Center near Mission Bay Thursday . . . but we all know it never should have come to this. There’s little excuse for the Celtics losing Game 6 at home. Brown and Tatum combined to take seven shots in the second half, two in the fourth quarter. Marcus Smart led the Celts with 15 shot attempts — which should never happen.

If these young Celtics lose again Sunday, they will wear this for the rest of their hoop lives.

It turns out that winning games is more important than winning quarters. The Celtics have to be kicking themselves for blowing this chance to put the Heat down and get some rest.

“We had our chances,’’ said Boston coach Ime Udoka, after watching his young team fall back into November/December bad habits.

The Celts trailed most of the night and fell behind by 13 in the second half before mounting a comeback and taking a lead on a Derrick White three (22 points off the bench) with 4:42 left.

That’s when things fell apart and Brown and Tatum failed to step up. It was 99-99 with 2:28 left when Brown went to the line and yanked a pair of free throws. After a 3-point play by Butler, Tatum turned it over. Then Horford missed a jumper. Suddenly, Miami led, 105-99, and all the air went out of the Garden. A Butler dagger sealed it (107-101) with 43.9 seconds left.

The Celtics were 8½-point favorites against the wounded, top-seeded Heat. Most of us thought the first five games made it abundantly clear that the Celtics are the better team. If not for ridiculous deficit quarters of 39-18 (Game 3) and 39-14 (Game 1), the series would have been over with Boston winning in four or five games.

“Everybody is on the same page and understands the opportunity we have in front of us,’’ Udoka said before the game. “We need to come out and be aggressive like we did in the last game . . . wear them down like we did in Brooklyn in Game 4 and against Milwaukee in Game 7.’’

This did not happen. The immature Celtics were flat at the jump.

Butler, who averaged only 9 points per game in Games 3, 4 and 5, came out gunning with 21 in the first half.

“Boston’s defense is so strong, sometimes you just need great players to make great plays,’’ said Miami coach, Erik Spoelstra. “He had the will that would not let us lose.’’

Boston was sloppy with the basketball, made only 6 of 17 shots and got outrebounded, 13-7, in the first quarter. It was exactly the kind of uninspired start the Celts had been hoping to avoid. Butler, who scored only 19 aggregate points in Games 3 and 4, looked like Michael Jordan and the Celtics played like they wanted a weekend trip to South Beach.

Boston didn’t get its first lead of the night until there were only three minutes left in the first half. Boston’s biggest lead was 4 points. The Heat controlled this game.

On the parquet floor.

“We’re frustrated, but if it was easy, it wouldn’t be us,’’ said White.

When the Globe’s Gary Washburn asked Tatum about his confidence level for winning Game 7, Tatum asked, “On a scale of 1 to 10?’’

“Yes,’’ Washburn replied.

“Ten,’’ said Tatum.

Swell.

But we all know it never should have come to this.