BOSTON — Take everything you thought you knew about the Celtics heading into the playoffs and set it aside. We all knew they would play hard and compete and that their defense would keep them in games. That was swell, but this is the playoffs and talent wins the day in the postseason.

That seven-game series against the Bucks when they were blown out twice, revealing what appeared to be a massive talent discrepancy against a mediocre 7-seed? Yesterday’s news. Those missing all-stars — Gordon Hayward and Kyrie Irving? Irrelevant. That 22-point deficit in the second quarter against the Sixers on Thursday night? They’ve seen worse.

Just remember one thing about the C’s. Throughout the year, they kept winning games. They won 16 straight after losing Hayward and they won down the stretch without Irving. They showed it again last night, beating Philadelphia, 108-103, to take a 2-0 series lead.

Every team talks about having a next man up mentality, but the Celtics actually live by that credo. When Hayward was lost for the season mere moments into the campaign, Jaylen Brown and Jayson Tatum took that opportunity and become foundational starters. When Irving was ruled out for the year, Terry Rozier stepped in and performed like an All-NBA guard.

When Brad Stevens called on Greg Monroe — who played all of eight minutes the last two weeks — he came through with a couple of big buckets. When no one else could get their shot to fall and they were in danger of being run out of their own building, Marcus Smart, of all shooters, buried shot after shot.

“When something requires guts, Marcus Smart is going to step up,” Stevens said. “We had nothing going in the first half and he kept us afloat early. He may miss some throughout the course of the season, but when these lights go on and he steps into that shot, I believe in him.”

Belief is a powerful thing, ephemeral and fleeting though it may appear. We all might be surprised by what the Celtics have done to Philadelphia in the first two games of their conference semifinal series, but they are not. This is who they are and this is what they do, especially at home where they have won all six postseason games.

There were several pivotal swings in Game 2, none bigger than their second-quarter rally. When the Celtics are at their best, they come at you in waves. A turnover on one end leads to a transition three at the other, and before you knew it we had ourselves a ballgame.

“Honestly I didn’t even realize we were down 22,” Al Horford said. “I knew we were down big, but what we kept talking about was like, we can’t get it all back at once. We need to just chip away and just go. I felt like we just kept fighting, chipping away and doing that and got some momentum.”

The second pivotal moment came in the third quarter when they created some separation and scratched out a lead. Tatum, who is growing into his destiny as a featured scorer for years to come, hit the Sixers with a barrage of moves both inside and out.

The brief moments of hesitation that held Tatum back during his rookie season are gone now in the postseason. He’s playing free and loose and giving the Celtics a dimension they haven’t had since Paul Pierce was working his magic at the elbow. (That comparison, as blasphemous as it sounded earlier in the year, is looking more and more prescient as the postseason rolls along.)

And their defense turned up the heat, cutting off passing lanes and closing off driving avenues for the Sixers to get to the basket. They turned Ben Simmons into such a non-factor that there were legit questions afterward about whether Brett Brown should have stuck with T.J. McConnell over the presumptive Rookie of the Year.

“It’s a tough decision, I admit it,” Brown said. “This whole playoff experience is something that I want our young guys and our star players to learn from and grow. The decision do you go with T.J. or do you come back to Ben Simmons, I come back to Ben Simmons. He’s had a hell of a year. I think he’s going to have to learn how to play in these environments and I’m coming back with Ben Simmons.”

Simmons put the blame on himself, saying he was overthinking and not playing his game. True, but there were reasons for his struggle.

Horford has done spectacular work as the primary defender and when he’s needed at the five, Marcus Morris has come in and roughed Simmons up whenever he had the chance. Behind them, the Celtics have been committed to getting back on defense and taking away his passing opportunities.

“I have said this before, they are elite guarding their own men,” Brown said. “I think there’s a physicality and switch-ability that they got apples for apples on many, many different matchups.”

The final pivotal moment came in the fourth quarter when the Sixers fought back and built a five-point lead. That’s when events took on a life of their own. Rozier shook loose for a pull-up three. Morris buried one of his patented no-no-no-no-yes looks. And then Horford, who was magnificent yet again in so many ways, blew by Joel Embiid for a layup with the help nowhere to be found.

The onus is now on the 76ers. They came into this series as the favorites, albeit without home-court advantage. They will have to win both games in Philly to turn this into a series, and they are fully capable of doing so. We saw glimpses of what they’re able to do and one would assume that they’ll be more of those moments on their home floor.

Assumptions, however, have gotten us into trouble in this series. What we know is that the Celtics are not to be taken lightly. Their talent, decimated though it is by injury, still runs deep. If you are going to beat them, you’d better make sure that you finish the job. The Sixers learned that lesson in Game 2. We should all take heed.