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Celtics forward Jayson Tatum sat at his locker soaking his feet in a large bucket of ice late Monday night as Al Horford, whose stall sits just two away, walked over.

“Playoff Al,’’ Tatum said, shaking his head admiringly. “That’s what I’m going to start calling you. Playoff Al.’’

Horford smiled.

“See,’’ he said to the 20-year-old rookie, “these games are a lot different than the regular season.’’

These young Celtics are realizing that a little more each day. Their season could have ended against the Bucks on Saturday, but it didn’t. Instead, it continued on to these conference semifinals against a 76ers team that surged into TD Garden having won 20 of its last 21 games.

This, many reasoned, was where the undermanned Celtics would finally run out of baskets and hope. And then it was announced an hour before game time that they would have to continue on without yet another star, as Jaylen Brown’s strained hamstring — despite his protestation — was deemed too strained for this game.

But if there is one thing this team has grown accustomed to during this increasingly stunning season, it is finding a way to win no matter who is available and who is not.

So without Brown, without Kyrie Irving, without Daniel Theis, and, of course, without Gordon Hayward, Boston blitzed the 76ers with a barrage of 3-pointers and surged to a 117-101 win that they mostly controlled throughout.

“People think we’re weak and we’re not as strong as a team because we don’t have certain players,’’ guard Marcus Smart said. “But we’re all professionals, we’ve all been here, we all work, and we all have the same goal in mind, and that’s to win. Regardless of who we have or don’t have, we go out there and play.’’

The good news for the Celtics was that they did have Playoff Al, they did have the rookie who was soaking his feet in the ice bucket, and they did have their brash young point guard, Terry Rozier, who showed up to the game wearing a Drew Bledsoe Patriots jersey.

All three were spectacular, and they mostly had to be for the team to have a chance. Rozier had 29 points, 8 rebounds, and 6 assists, Horford added 26 points and 7 rebounds, and Tatum had a career-high 28 points. The trio combined to go 29 for 46 from the field and 15 for 16 from the foul line.

Despite those flashy numbers, though, the Celtics were most proud of how they kept Philadelphia doing something similar against them.

“We all got together yesterday, and we had a plan,’’ Rozier said. “Our plan was ways that we could stop these guys, no matter who’s on the court.’’

The 76ers made just 5 of 26 3-pointers and were kept at an arm’s length or farther throughout the game’s final 30 minutes.

Joel Embiid had 31 points and 13 rebounds, but he did not receive nearly enough help.

It could have simply been viewed as an off night for a team that had been idle since finishing off its series against the Heat last Tuesday. But coach Brett Brown made it clear that the struggles were not simply self-inflicted.

“To look at this game, defensively, offensively, this isn’t who we are,’’ he said. “This is a very poor game for us, and I give the Celtics a lot of credit for producing that.’’

Despite the powerful performance, Celtics coach Brad Stevens said his team will need to do more moving forward.

He said the defense was not as good as it was in any of the last three games against the Bucks, and that slice included one game in which Milwaukee made 50.7 percent of its shots.

“There were parts about it that were good, but we have to clean up quite a bit,’’ Stevens said. “They exposed us in a lot of areas.’’

Of course, the return of Jaylen Brown could help. About 90 minutes before the game, he was on the phone with members of the team’s medical staff, looking for a way to play. But he was eventually ruled out.

The fact that such a long time passed before the final decision was made figures to be a good sign for Brown moving forward, particularly with two days off before Thursday’s Game 2.

“One of the things when I talked to him I was like, ‘Hey, we obviously really need you out there, but we need you for the long run and we need you to be healthy and to feel good,’ ’’ Horford said.

And if Brown is not back, there is no doubting this team’s confidence in its ability to push on without him.

“This isn’t our first rodeo with things like this,’’ Smart said. “We’ve had injuries before. We’ve come back from deficits you shouldn’t be coming back from. So this team, we’re so high on each other, if things are thrown at us, we just look at it as another challenge. When it’s time to get out there, we just go out there and do it.’’

Before Game 1, Brett Brown acknowledged he was unsure if the long layoff could hurt his team.

He had tried to balance practice time for Embiid — who already had a lengthy absence due to his orbital fracture — with rest for the others. But he just did not know how it would all look.

Then the game began and Philadelphia fired up one errant shot after another. At the start, most of them were open, but then Celtics made them more difficult, and the misses kept coming anyway.

“I don’t know if the time off hurt us,’’ Brown said, “but it looked like we were playing a good team today.’’

The 76ers overcame a slow start and actually took a 29-27 lead on an Embiid jumper with 9:32 left in the second quarter.

But they faded quickly after that.

With 3:12 left, Tatum drilled a catch-and-shoot 3 from the left corner, and two minutes later slid along the baseline and put in a two-handed dunk, helping the Celtics take a 56-45 lead to halftime.

Embiid found a rhythm inside early in the third quarter, but the Celtics were resilient, and they were also happy to receive some long-range contributions from unlikely sources.

Smart, who had been 0 for 11 on 3-pointers in this postseason, finally hit one midway through the period, and then Aron Baynes, who had just four regular-season 3-pointers in his career, hit his second of this game, helping Boston stretch its lead back to 72-58.

With 40.9 seconds left, Smart soared and grabbed an offensive rebound over Embiid and then converted a circus shot as he was fouled. The free throw gave Boston an 87-70 lead, its largest.

Adam Himmelsbach can be reached at adam.himmelsbach @globe.com. Follow him on Twitter @adamhimmelsbach.