In the visiting locker room at Boston’s TD Garden before Thursday night’s Game 2, there were signs everywhere.

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Some were literal, like the one Sixers staff had hung to the left of the locker room door: “Philly Hard,” emblazoned in black over an image of the Liberty Bell. Or the message scrawled in all caps on the top, right-hand corner of the whiteboard: “MAKE THEM FEEL US,” with the third word in bright red.

Some were figurative, like Ben Simmons addressing the media pregame and saying that the team that fans saw in Game 1, the one that lost by 16 to the undermanned Celtics, wasn’t the real Sixers. “It was frustrating losing that game, because that just wasn’t us,” the Rookie of the Year favorite said. “I’d rather lose and know we played our game, and played the way we’re supposed to play.”

Asked whether the Sixers expected a similar game plan from the Celtics on Thursday, the big Aussie responded with a verbal shrug if ever there was one: “I have no idea.” What the Celtics did or didn’t do was beside the point. Entering the series, Philadelphia had won 20 of its past 21 games, and was determined not to overreact to one loss.

The Sixers were going to stick with what got them here in the first place. That adherence to the (ahem) process resulted in a 108-103 loss, and a trip home to Philadelphia facing a 2-0 series hole.

After player introductions Thursday night, the Celtics played a video clip of Joel Embiid in the Game 1 postgame presser to pump up the crowd.

Reporter: “Talk about the atmosphere tonight. It seemed pretty loud.”

Embiid, deadpan: “It was OK.”

It had the desired effect (Crowd: Booooooooo!) ... and then Philly won the tip and took off.

Shortly afterward, the Sixers ran J.J. Redick off of a screen, and he splashed a 3. A few possessions later, Redick got free for a layup. And then, in case anyone hadn’t caught wind of the pattern, Redick ran off a screen and drilled a 3. Brad Stevens called for time with the visitors up 8-0, the crowd quiet.

True to their messaging, the Sixers were physical. They popped the nets regularly, clung to Boston’s shooters like a cheap suit, and hit the offensive boards hard. Embiid looked like a 7-foot Peter Parker, with misses caroming off the rim and zipping to his outstretched hand as if on a web. In the first quarter, the Sixers had 10 assists on 13 made buckets and outrebounded the Celtics 18 to 10. At the end of the period, they led by nine.

The lead grew to 22 in the second. Increasingly, Philly looked like the team everyone had expected to see in this series. And then, like a junkball pitcher who’s running out of ideas, Stevens threw the Sixers an eephus pitch. Desperately seeking offense and physicality, he inserted Greg Monroe, and beyond belief, the decision worked. The big man, picked up off the scrap heap midseason after being tossed aside by the Suns, battled with Embiid on the block and the Celtics scored a couple of buckets.

Simmons, a pickup truck personified, was called for an offensive foul when Marcus Smart slid into his path midway through the second quarter. Monroe drove the lane and scored. On the other end, Redick missed a jumper from the free throw line and Terry Rozier pushed the ball upcourt, punching into the lane and kicking to Jaylen Brown in the right corner for a 3.

All of a sudden, the hosts were off and running.

“Once we started getting stops and pushing the ball, it was history from there,” Rozier said postgame.

The Sixers—so confident initially, so full of energy and so crisply efficient (31 points on 27 shots in the first, including 4-for-6 shooting from 3)—had no answer, as the NBA.com lead tracker neatly illustrates:

The Celtics never ran away with Game 2. But once they realized that the Sixers’ best shot hadn’t been a knockout blow, they decided to just win the fight on points and eked out a five-point victory. Boston is without injured stars Kyrie Irving and Gordon Hayward (and important reserve big man Daniel Theis); it’s downed Philly in consecutive games all the same.

“I think the Celtics’ defensive intensity went to a higher level,” Philly coach Brett Brown said postgame. “I think the way Boston is guarding us in general is something that I respect. They are elite guarding their own men. And I think there’s a physicality and a switchability [to their roster], that they’ve got apples for apples on many different matchups.”

To the untrained eye, physicality appeared to bother the Sixers, especially Simmons. At one point late in the fourth quarter, he called for the ball on the block, got it, tried to bull his way to the basket, stumbled, and missed. A little later, he got the ball on the baseline with fellow Aussie Aron Baynes on him. That should’ve been a speed mismatch; instead, Simmons took a couple of hesitant dribbles and hurled a wild cross-court pass to a cutting Dario Saric as the shot clock ticked down.

“I think [my problem] was mainly what I did to myself,” Simmons said of his 30-minute, 0-for-4-shooting, one-point, seven-assist, five-rebound, five-turnover line. “I think mentally I was thinking too much. I wasn’t just out there playing.”

He even gave the Celtics prime bulletin-board material, should they choose to use it moving forward: “The way the Heat played [in the first round, what the Celtics did] was nothing ... The Heat were on another level.”

“You knew it was gonna be a close game,” Brown said at his postgame press conference, dressed in a subtly pinstriped blue suit. “The notion that we were gonna maintain a 20-point lead and walk out of the Boston Garden wasn’t on my mind.”

After Game 1, Brown said the Sixers wouldn’t make any drastic changes. And after they fell into a 2-0 hole, he echoed the same sentiment: “If I had to do it again, I would do the same thing. I would have the same people in the game.”

That insistence on staying the course speaks to part of Philadelphia’s problem in these first two games, beyond a young team just experiencing all of this adversity for the first time. When the going got tough, one team pulled out all the stops to try to get things going. The other had already done everything it could think to do. What matters is demonstrating the ability to absorb new information and, if necessary, create a new plan on the fly.

At the start of the fourth, backup guard T.J. McConnell gave the Sixers a boost. He scored a couple of buckets and, later, sneaked inside to swipe a rebound and toss the ball out to Robert Covington for a 3. When he subbed out with five minutes and change remaining, the Sixers had the lead, 93-91.

But Brown wanted to get Simmons back in the game, so McConnell had to come out.

“It’s a tough decision. I admit it,” Brown said. “This whole playoff experience is something that I want our young players and our star players to learn from and grow. The decision, do you go with T.J. still or do you come back to Ben Simmons? I’m coming back to Ben Simmons.

“I think he’s had a hell of a year. I think he’s the rookie of the year. I think he’s gonna have to learn how to play in these environments. And I’m going with Ben Simmons.”

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That’s understandable, and it’s easy to second-guess with the benefit of hindsight. But in this instance, it felt like the wrong decision in the moment. It is important to help young star players learn about playoff basketball, but it’s also important to recognize what’s working and what isn’t.

The Celtics have never lost a series in which they held a 2-0 lead. And NBA teams that lost the first two games of a seven-game series on the road are 15-for-236, for a winning percentage of 6.36.

“We know what we have to correct,” Embiid said postgame. “Honestly, I’m not too scared. Because we know what we’ve gotta do to be able to win games. They’re a very good team and they’re well coached. We’ve just gotta execute our game plan.”

There’s truth to those words, and, with home-court advantage on their side in the next two games, the Sixers could easily even the series starting with Saturday’s Game 3. But even if they execute their plan, they’ll have to demonstrate the ability to digest and adapt to their opponent’s in-game adjustments if they want to prove they’re a true contender.

Until then, they’re still processing.