This should have been a lay-up. A get-to-the-next-round-free card.

The Celtics should have been on their way to a date with Philadelphia after eliminating the short-handed Hawks.

It should have been one series down, one opponent down, with an easy path ahead to the promised land.

Only the Celtics weren’t able to finish off the series. They blew the bunny, as they were taken down at TD Garden, 119-117, by the Hawks in a game they could have easily knocked them out, and moved on.

And now there are more questions than answers. There is more doubt in the Celtics than confidence after taking their foot off the gas, and letting the Hawks back in the series.

It’s almost unforgivable for a so-called championship-driven team to lose a potential knockout game like this.

The road is practically paved for them to make it to the NBA Finals. And yet, they shot themselves in the foot, losing a potential series-clincher to a Hawks team playing without star Dejounte Murray, who was suspended for the game after bumping an official at the end of Game 4.

Even with a golden path ahead, it’s still hard to believe in the C’s given their inability to hold leads, and close out games. It’s hard to trust this team because that’s how it’s been most of the season.

Watching them Tuesday night, they just don’t look like they’re ready to win a championship, much less get there with an easy road.

They pretty much led the entire game before crumbling in the fourth quarter.

Joe Mazzulla had no answers. He was out-coached by Quin Snyder down the stretch.

It also didn’t help that Jayson Tatum was ice cold (8-for-22, 1-for-11 from three-point range) and landed a technical in the closing minutes.

Jaylen Brown, who was sensational with 35 points, couldn’t hit free throws (1-for-5), especially down the stretch, missing a pair with just over seven minutes to go in the fourth quarter, with the Hawks creeping closer.

Marcus Smart, meanwhile, couldn’t stop picking up fouls in the fourth quarter, while Al Horford only mustered three points.

Add in the fact that no one could guard Trae Young, who hit the game-winning three with 1.8 seconds to go. Mazzulla had Brown, not Smart or Derrick White, on Young (game-high 38 points) during that final possession.

The Celtics did have a chance with 1.8 seconds to go, but Tatum’s hurried heave missed and there was a sense of deflation all around TD Garden.

“We can’t change anything,” Tatum said following the loss. “We just gotta do a really great job of moving on. It’s as simple as that.”

Only nothing is simple for this team.

The C’s were up by 13 with 9:32 to go and later by a dozen with less than six minutes to go and still couldn’t close out the Hawks.

Instead of going for the throat and putting the Hawks away, the offense went stagnant. Mazzulla said they simply lost their pace.

“I think we’ve just got to execute,” Mazzulla said following the loss. “I thought we got some good shots, but I just didn’t think we executed the way we normally do down the stretch.”

Yup, they had the collective collar around their necks, at least, that’s how it looked.

This should have been NBA Finals or bust time for the Celtics.

Instead it’s worry time, even up 3-2 in the series.

If they don’t advance, it’s on them. More to the point, if they don’t reach the point where they have a chance to address “unfinished business” — their stated playoff motto — it’ll be hard to blame anyone else.

“If you don’t win, you don’t execute, then you lost your poise,” Mazzulla said. “I don’t know if we were trying to do too much, I just didn’t think we were poised and just didn’t execute well.”

At the outset, the Celtics had a wounded opponent on the ropes with one win needed to advance. And the game was at home, to boot.

They knew they needed to finish off Atlanta without adding more games to the series. They knew that was key.

When it comes to the playoffs, efficiency means a lot. The Celtics know that more than anybody.

Last year, they kept adding extra games into the mix before eliminating opponents.

They did get to the NBA Finals, but didn’t have a lot left in the tank to deal with Golden State.

After going up 3-1 in the series after beating the Hawks in Atlanta, Tatum brought up that very point. He said the Celtics had learned their lesson from last year’s experience. They knew it behooved them to finish off the Hawks, and get some rest, before taking on Philadelphia.

Guess the message wasn’t received.

On Tuesday night during the potential close-out game, they blew countless leads. They missed free throws. They kept heaving threes with a double-digit lead. They didn’t play sound defense. And now they’re headed back to Atlanta for Game 6 on Thursday after giving the Hawks life.

“We pride ourselves as players (making) the right play and adjustments,” said Brown. “At times, we leave that up to the coaches to make that decision…We gotta be better overall as a team, coaching staff, & players on getting organized in those moments, whether we call timeouts or not.”

How can they flip the script?

“They know,” said Mazzulla. “Just focus on what we did well, where we can get better, and just have an awareness to those details on both ends of the floor.”

That was the mission prior to Game 5, as well. Only they continue to do things the hard way.

Given how the dominoes are falling, specifically who’s hurt, and who’s not, the Celtics needed to win, and rest, as opposed to head on a plane and contemplate a turnaround in Game 6.

Sixers star Joel Embiid is injured, and has a huge question mark over his head in terms of his readiness for the next series.

He’s battling a knee injury, which is never good. But now he gets extra rest.

Add in the top-seeded Bucks being on the ropes, trying to win with Giannis Antetokounmpo also wounded, toughing it out with a bad back, and the C’s couldn’t be in better shape.

If they do ultimately advance, they might not even have to face the Bucks, who are down 1-3 to the Miami Heat.

So it was all falling into place. Only the Celtics couldn’t do their part in a timely fashion. Instead, they’re sabotaging themselves and their journey ahead.

Said Mazzulla: “We have another opportunity on Thursday, so I think it’s a matter of having perspective on the situation and then learning and then just growing through it. So we had an opportunity, and now we have another one. So now we just have to focus on that.