Celtics forward Jayson Tatum, left, holds his palm to his forehead while sitting on the bench teammates during the second half of Tuesday’s playoff game at TD Garden in Boston. Philadelphia defeated the Celtics, 115-103. AP Photo/Charles Krupa

So-called championship-driven teams always know when to rise to the occasion. They know when it’s time to make their move in a game, but especially in a series.

Based on the evidence at hand, the Boston Celtics are a far cry from being championship caliber right now.

In Game 5 of the Eastern Conference semifinals against the Philadelphia 76ers on Tuesday night, with the series tied 2-all, the Celtics needed to bring their “A” game.

They needed to impose their will and take command of the series.

They could no longer afford to dilly dally, then turn on the jets at crunch time. They should have learned their lesson long before this. They should have already realized that tactic didn’t work in Game 4, as well as several other games during this postseason run.

Talk about embarrassing. With 3:55 to go, down by 20 points, Coach Joe Mazzulla emptied the bench and put in the subs. Even worse, the team was booed off the court at different times by a fandom that rarely is driven to that end.

At least the scrubs showed a little more urgency during the 115-103 Game 5 loss.

Bottom line: The Celtics have made their bed. Blame Mazzulla if you want, but ultimately, it still comes down to a team that’s underperforming during crunch time. They basically took a powder during the worst possible time.

“That was the first game of the playoffs that we didn’t play well, in my opinion,” Mazzulla said following the loss. “So we can’t lose our perspective. We played really good basketball, and that was our first really, really bad game of the playoffs.

“So it doesn’t come at necessarily the best time, but we just have to shift our perspective and get ready for the next game.”

It’s hard to believe it’s gotten to this point, but the Celtics are down 3-2 headed to Philadelphia for Game 6 on Thursday night.

In arguably the biggest game of the year, a game Celtics guard Malcolm Brogdon dubbed a “must-win” at the outset, the Celtics were wiped off the court by a hungrier team.

What went wrong, Celtics guard Marcus Smart was asked after the loss?

“Everything. Everything went wrong,” Smart said. “They made every right play. They made every hustle play. Everything went wrong for us that can go wrong.”

Maybe the Sixers are simply a better team – a concept that didn’t seem possible at the start of the playoffs. Maybe the Celtics, like the Bruins, can’t cut it when the stakes are the highest.

Wasn’t Joel Embiid 1-10 against the Celtics in the postseason heading into Game 4? Aren’t the Celtics so much better on paper?

Well, the Celtics needed their stars to shine, and once again, so many of them didn’t deliver in the pivotal game of the series.

As good as he is, Jaylen Brown just can’t hit a free throw with the game in the balance, and it’s been a killer. The Celtics had trimmed the lead to 11 with 9:16 to go. Tyrese Maxey hit a 3-pointer to put it back up to 14, but then Brown clanged a pair of free throws. The Sixers never looked back after those misses.

While Brown had 24 points, he was 3 for 8 from the line. He was also saddled with early foul trouble. On the whole, the team was 25 of 34 (73.5 %) from the line, which is unacceptable at this time of year.

It was death by missed free throws.

Even more of an embarrassment?

Al Horford.

He was the bagel man. He went 0 for 7 from 3-point range – 0 for 6 in the first half, which tied him for the most attempts without a make in a half in Celtics postseason history.

Super sixth man Brogdon wasn’t much better, hitting just 3 of 9 shots.

Championship teams can’t win with efforts like that.

As for Jayson Tatum, he finished with a game-high 36 points. Don’t let the numbers fool you. As in Game 4, he couldn’t hit the side of a barn door for much of the first half. He had two points in the first quarter, going 0 for 5 from the field.

His numbers got better because he decided to go to the hoop, given his outshot shots weren’t failing him. Unlike Brown, he delivered from the free-throw line, hitting 11 of 13 attempts.

The Celtics were supposed to have a deadly one-two punch with Tatum and Brown. One, or the other, or both, delivered constantly during the regular season. And at different points during the playoffs, they have come through. But between foul trouble and being absent on the scoresheet for a quarter or a half, they aren’t getting the job done.

After a year to grow and mature, and learn from the experience or last year’s NBA Finals, both Tatum and Brown seemed ready to carry the Celtics. Only it hasn’t happened in the way many had hoped.

In this series, Embiid is doing a better job of carrying his team, and delivering knockout blows. He had 33 points, and was aided by Maxey (30 points), who was lethal with the 3-point shot.

In Games 1 and 4, it was James Harden who delivered.

As Sixers Coach Doc Rivers predicted prior to tip-off, Game 5 would come down to “making shots … it’s a make-miss league, that’s the bottom line.”

The Sixers made 50.6 % of their shots. The Celtics connected on only 39.8% (33 of 83). They were woeful from 3-point land (31%), while the Sixers were 40%.

“The energy wasn’t right. It could have been better, way better,” Smart said. “We know that, we understand that. And tonight speaks volumes. It shows that, if you’re not ready, you’re going to go come out, any given moment, this can happen, so you tip your hat off and move on to the next one.”

It also speaks volumes about a team that claims it has “unfinished business” and wants to win a championship, after losing last year in the NBA Finals.

It speaks volumes about a so-called championship-driven team.