The Celtics were booed off their home floor. They were disappointed as they headed to their locker room.

But the C’s weren’t sulking after their embarrassing Game 5 loss to the 76ers suddenly put their season in jeopardy. They’ve been here before. Last season, in the same exact spot in the second round, the Celtics lost a late lead to the Bucks and dropped Game 5, putting them in a 3-2 series hole.

Of course, the rest is well-documented. The Celtics won Game 6 in Milwaukee, then Game 7 in Boston to advance to the Eastern Conference Finals. Now, they have to do it again. As frustrating as the Celtics may be, routinely putting themselves in unideal situations, they know what this moment requires as they enter Thursday night’s do-or-die Game 6 in Philadelphia.

“The brutality of it. It’s a true dogfight, scratching and clawing, biting, blood, everything,” Marcus Smart said. “And if you’re not willing to pretty much get dirty, if you’re not willing to bleed, if you’re not willing to break something, willing to tear something, going hard, then you shouldn’t be on that court because that’s what it is. That’s what the playoffs are about. Hopefully you stay safe but that’s the mentality. You gotta go, you gotta be willing to risk it all for these games. And that’s the mentality we gotta have. …

“Been there before,” Smart said. “It’s one game at a time. So they’re feeling good. We got to go into a hostile environment and we got to just go take it. It’s not going to be easy. It’s going to be a dogfight. And it’s, who wants it more?”

Inside a silent Celtics locker room after Game 5, there was a quiet confidence. Jaylen Brown brushed off the comparison to last season’s Bucks series – “last year is over,” he said – but knows the C’s have been resilient after bad losses all season. They’ve responded when their backs have been against the wall.

The Celtics have left themselves no room for error because of how this series has unfolded. They dropped Game 1 at home against the Joel Embiid-less Sixers. They failed in crunch time in Game 4. Now they have to go into Wells Fargo Center – where a raucous Philadelphia crowd will be hoping to will the Sixers to their first berth in the Eastern Conference Finals since 2001 – and escape with a win.

After reaching the NBA Finals a year ago, the Celtics are more talented this year. Their path back to the Finals has been cleared for them. Failing to even get out of the second round would be a catastrophic failure.

“We’ve been in this position before,” Al Horford said. “It’s not ideal. But we still have an opportunity. We understand what it takes to go on the road on Thursday.”

Celtics coach Joe Mazzulla isn’t necessarily the type for motivational speeches. But he’ll have a message before Game 6.

“Play free, play together, get connected, play physical and leave it all out there,” Mazzulla said.

Jayson Tatum said the routine won’t change much before Game 6. But when they step on the court, the Celtics clearly need some adjustments.

It starts with Tatum, who’s been woeful in the first quarter in the Celtics’ Game 4 and 5 losses. He went 0-for-5 in the first and missed his first eight shots in Game 4. He went 0-for-5 in the opening period of Game 5 and missed his first six shots.

In Game 6 at Milwaukee last season, Tatum erupted for 46 points in a signature playoff performance. The C’s don’t necessarily need that to leave Philadelphia with a victory, but they certainly need a better Tatum.

“I think just instilling his confidence,” Mazzulla said. “I think when he gets off to a good start, we’re a better team. Also understanding it doesn’t necessarily have to come from points. I think when he’s at his best, he’s just navigating both offensively and defensively.”

The Celtics thrive when they’re running and in transition, and it starts on defense. They never got into a rhythm in Game 5 because they couldn’t get stops and kept committing fouls, most often on Embiid.

The C’s, of course, have to hit their open 3-pointers. Mazzulla said they went 7-for-28 from deep on open to wide-open looks. That’s a recipe for disaster for the Celtics, who have proven to be almost unbeatable when they’re hot from long range and vulnerable when they’re cold. If they can lock down defensively – which has been inconsistent in this series – the Celtics’ offense will be better for it.

“Get stops. I think it starts with that. It starts with our defense,” Mazzulla said. “If you go back to when we’re playing at our best, that stretch of Game 4 here on the road, we were down eight to up six. We hit three 3s and it was because of the cross matches. It was because we drove and kicked and we knocked them down.”