We’ve heard this sage advice before: After a stunning Celtics’ loss to Hawks, anything can happen.

© Jim Davis/Globe Staff Trae Young (left) and the Hawks were celebrating his 3-pointer with 1.8 seconds left as Jaylen Brown headed for the bench.

Perhaps as soon as Thursday, Boston fans will be able to look back and laugh. Maybe in six weeks we’ll remember this one as the night the immature, still-without-rings Celtics finally woke up and realized they’re not as good as they think they are. Maybe this will prove to be the epiphany — the moment in which the Celtics finally understood something so simple and so huge.

But this is not that moment. Right now, in real time, this can only be described as one of the greatest gag jobs in Boston sports history. Call it Atlanta’s revenge for 28-3. It was a collapse worthy of the pre-2004 Red Sox. At this moment, the 2023 Celtics are Parquet Posers.

Graced with the presence of the Tomato Can Hawks (41-41), the mighty Celtics — Vegas favorites to win the NBA championship — Tuesday coughed up a 13-point lead (at home!) in the final six minutes of a potential Game 5 clincher and lost to the undermanned Hawks, 119-117. Trae Young’s calm, 30-foot pull-up with 1.8 seconds left stunned the Celtics and NBA America. The unraveling Celtics never got another shot off.

Game 6 will be played Thursday night at State Farm Arena in Atlanta. The Celtics probably will advance and we’ll forget about all this. They are clearly more talented than the Hawks. But they have also demonstrated that they lack direction and leadership. They have toyed with Atlanta for most of this series, but now it is indeed a series and that’s all on Boston. If these Celtics fail to win a championship this spring, when their path has been so nicely cleared, they will go down as one of the most disappointing teams in Boston sports history. You’ll be OK to put them alongside the 2003 Red Sox and the 2007 Patriots: front-runners, then failures.

The Celtics have declared this playoff run as a mission to take care of “Unfinished Business.’’ And they were handed a gift when the Hawks emerged as their first-round opponent. Boston led by 32, then 22, in easy Game 1 and 2 victories. There was a mild upset in Game 3 when the Hawks shot the ball like Sons of Maraviches. Order was restored when Boston won Game 4. Tuesday at the Garden was supposed to set up Celtics-Sixers Saturday in the conference semifinals. The Hawks didn’t have 25-point-per-game guard Dejounte Murray (suspended for bumping an official Sunday) and were 13-point underdogs.

As in three of the first four games, the Celtics led pretty much from wire to wire, keeping the Hawks at bay until the final minute and a half. Jaylen Brown (35 points) was electric. Boston led, 109-96 with six minutes left. Then Jayson Tatum (1 for 10 on threes), Brown, and Marcus Smart all committed turnovers and Smart was slapped with an offensive foul as the Hawks surged back.

I mean, how do you gag at home against these guys?

Even the Hawks didn’t think they were going to win. State Farm Arena in Atlanta had scheduled a Janet Jackson concert for Thursday night, which now has been pushed back to Friday.

The Hawks finally took the lead on three free throws (one thanks to a technical foul on Tatum) by Young with 1:39 left, then won it when the indomitable Young calmly drained a 30-foot pull-up parabola with 1.8 seconds remaining. The Celtics called time, but failed to get a shot off, butchering two in-bounds plays in the final seconds.

It can’t be fun for Boston’s rookie, 34-year-old coach Joe Mazzulla to have this happen on the same day Ime Udoka was named head coach of the Houston Rockets. Mazzulla acknowledged that his team “lost their pace” down the stretch.

That’s polite phrasing for a six-minute train wreck against a beatable opponent at home.

Oh, and let’s not forget that it was in Boston’s interest to get this series over with and bring on the 76ers as soon as possible. Philly’s MVP-designee big man Joe Embiid messed up his knee in Game 3 in the 76ers’ sweep of the Nets and now will have extra time to heal before the start of the conference semifinals.

We have assumed all along that these Celtics would advance, but nothing is certain anymore. The only thing that is certain is that the Celtics, until proven otherwise, are front-runners and underachievers.

They will probably still win this series.

We keep saying that.

But it should never have come to this.

It should have been a sweep. Or a gentleman’s sweep. But now it is a series. And you know what Kevin Millar says:

Anything can happen.