We were overly optimistic, weren’t we?

We expected the Boston Bruins, the best team in the National Hockey League during the regular season, to hoist the Stanley Cup this month.

We expected the Boston Celtics, the second-best team in the National Basketball Association, to dribble to their 18th championship, especially after top-seeded Milwaukee exited in the first round of the playoffs, the Celtics got by Atlanta and Philadelphia and then welcomed eighth-seeded Miami to TD Garden for the Eastern Conference Finals.

Alas, our optimism was misplaced. We all know how the Bruins playoff run turned out. An epic collapse in the first round and a Game 7 overtime loss to the Florida Panthers on the TD Garden ice.

And now we know how the Celtics ended. An epic collapse in the Eastern Conference finals, culminating in a stunning 103-84 blowout loss in Game 7 on the TD Garden parquet.

Can you believe the Bruins and Celtics, charter members of the NHL and NBA and organizations dripping with tradition, lost Game 7s to a pair of teams from South Florida?

Nobody — not even Pat Riley — could have predicted the horror show that occurred Monday night. A Miami victory was not out of the question. The Heat did bounce Milwaukee and did win the first two games of this series in Boston, right? 

But a blowout in Game 7? In Boston? In front of a pumped up, sold out house? Beneath those 17 championship banners and all those retired numbers of Celtics heroes past? No way!

But that’s exactly what happened because the Celtics left their hearts in the locker room. After fighting and clawing their way back from a 3-0 abyss, after earning a winner-take-all showdown in their building, in their city, in front of their fans, they turned in a performance for the dark ages. The 103-84 final was no indication of how poorly the Celtics played.

All-NBA Jayson Tatum rolled his ankle on Boston’s first possession and struggled to score 14 points on 5-for-13 shooting. Jaylen Brown, the second-team All-NBA performer, played his worst when his team needed his best. He finished with 19 points on 23 shots and had as many turnovers, eight, as field goals. Miami’s five starters had only seven turnovers.

The Celtics problems went way beyond the failure of their two stars. Encouraged by their rookie coach, Joe Mazzulla, they were a three-point shooting machine this season. When the threes went in, they looked great, unbeatable. But when they caromed off the rim or the glass, they looked horrible. 

Monday night they were horrible. They missed their first 12 shots from three-point range. They put up 42 three-point attempts and made only nine. That’s 21 percent, folks. Not good. In Game 6 they launched 35 treys and sank only seven — 20 percent— but still won thanks to Derrick White’s miraculous buzzer-beating put back of a Marcus Smart miss.

Their worst three-point shooting games were their last two.

If the Celtics had a Plan B, it’s still s secret. Their answer to missed three-pointers was to take even more three-pointers. TNT’s Charles Barkley castigated the Celtics for being “undisciplined” and “un-fundamentally sound.” He said the Celtics “play stupid.” 

On this night, he was right.

Boston’s defense was ineffective. The Celtics tried to stymie Jimmy Butler, but he still scored 28 points. Caleb Martin added 26 points on terrific outside shooting. Miami made 14 of 28 threes.

In the end, after losing a heartbreaker at the buzzer, Miami came back to Boston, played with grit, determination and toughness and beat the Celtics in Game 7 exactly one year after Butler missed a late three-pointer in Miami that enabled the Celtics to win the Eastern Conference title.

For their part, the Celtics showed no heart and no mental toughness, which was unsurprising. They lost three home games in this series. Three! Championship teams do not lose three home games in a playoff series..

So what’s ahead for this team? A lot of soul searching and a lot of questions. Is Joe Mazzulla out as coach? I doubt it, unless the players have lost their confidence in him. Brad Stevens put him in a difficult position after the team suspended Ime Udoka last fall and should give him another chance and a veteran assistant or two for next season.

Does Jaylen Brown get the $290-million supermax contract he earned for being named to the All-NBA second team? That’s a tough one. Jayson Tatum wasted no time in saying he wants Brown back.

Does Marcus Smart return as the starting point guard? His defense slipped this season, and his ball-handling and playmaking are erratic.

Is their supporting cast strong enough?

Let’s remember this team won 57 games and two playoff series this season and came back from a 3-0 deficit to force Game 7 in the conference finals. The challenge for Stevens is to find the missing link needed to win that 18th championship. The question is how much is he willing to give up.

But for now, cancel the duck boats. There will be no championship parades in Boston this spring.

Mike Szostak can be reached at mszostak@ripr.org.

Mike Szostak covered sports for The Providence Journal for 36 years until retiring in 2013. His career highlights included five Winter Olympics from Lake Placid to Nagano and 17 seasons covering the Boston...