After Game 1, Sixers need fast growth from Joel Embiid, Ben Simmons | David Murphy

BOSTON – How fast can the Sixers grow up?

That's the question that looms after Game 1. The question might not be entirely fair, but neither is the task that the Sixers are asking their two young stars to accomplish this postseason. That they entered this best-of-seven series as the favorite to advance to the Eastern Conference semifinals is a testament to the raw athletic potential of Ben Simmons and Joel Embiid. That they now trail, 1-0, after a 117-101 loss is a reminder that said potential still has a ways to go before it is fulfilled.

No doubt, there are a lot of things that the Sixers did wrong on Monday night that do not fall directly at the feet of their two centerpieces. It is difficult for a basketball team at any stage of maturity to win a game in which it shoots 5-for-26 from three-point range. JJ Redick, Ersan Ilyasova and Robert Covington are three of the Sixers' more experienced hands, and they combined to miss 12 of their 14 shots from deep. The Celtics were ready, and the Sixers weren't, and everybody from Simmons and Embiid to head coach Brett Brown admitted as much. Either Brad Stevens had a better plan, or his players executed it better. Either way, the butt-kicking they delivered was so thorough that it transcends individual blame.

Still, the Sixers were supposed to have the advantage in this series in large part because of the Celtics' inability to match up with Embiid and Simmons. This time of year, it often comes down to the team with the player who is capable of taking over games, and the Sixers have the series' two best players in that department. Maybe that was a conscious thought of theirs after they entered the locker room trailing, 56-46. The third quarter belonged mostly to Embiid, who spent the period exploiting the advantage in lateral quickness that he held over Celtics big man Aron Baynes. He finished with 31 points on 12-of-21 shooting and kept the Sixers in the game.

But on the defensive end of the court, the Sixers struggled with missed assignments and blown coverages. Maybe it was the rust, but maybe it was the kind of thing we've forgotten to expect out of a team this young.

"Defensively, we weren't like ourselves," Embiid said. "Next game we just have to be aggressive and play tough defense."

The Celtics made it look easy, chiefly in the form of 12th-year big man Al Horford, who spent most of the night playing at the four spot to accommodate the presence of veteran big man Aron Baynes, who was charged with banging down low with Embiid. Throughout the night, the battle-hardened star displayed a command of his surroundings that can only come with time, scoring 26 points and dishing out four assists while shooting 10-for-12 from the field and 2-for-3 from three-point range.

Horford was also a crucial piece of the varied defensive looks that Stevens and the Celtics threw at Simmons. Early on, they unloaded a revolving series of defenders on the Sixers rookie, first Marcus Smart, then Horford, then Jayson Tatum, with Baynes lingering in the paint to provide help against a potential drive. At one point, this series of events left the seven-foot Embiid guarded by the 6-foot-4 Smart, but the whole sequence seemed to leave the Sixers out of sync, and Embiid could not capitalize. Tatum's first tangle with Simmons ended with him skidding across the floor after he beat Simmons to the spot and lost the battle of physics. Again, though, the Sixers could not capitalize, as Simmons dribbled into the lane without a defender in front and then unleashed a wild pass out of bounds after the Celtics' help collapsed on him.

There were plenty of moments when the raw talent of the Sixers young core prevailed. Early in the fourth quarter, Simmons grabbed a rebound and threw the length of the court to Embiid, who threw down a two-handed dunk that cut the deficit to 87-77. But a couple of minutes later, Embiid turned the ball over while deep in the low post and, on the ensuing possession, the Sixers left Rozier unguarded and could only frantically close out as he buried a three-pointer that pushed the Celtics' lead to 91-79. By the time it was over, the Celtics had connected on 17 of their 35 three-point attempts.

"I think we are just trying to get our legs back under us," Simmons said. "It's a different pace once you are actually on the floor and not practicing. It's one of those things where we have to get back on track and stick with the team."

Granted, Boston isn't exactly long in the tooth. One of the most electric players on the court on Monday night was the guy the Sixers essentially traded for Markelle Fultz on draft day last June. Tatum, whom the Celtics drafted at No. 3 with the Sixers' original pick, scored 28 points. Fultz did not play. Smart, Brown, Rozier — all are under the age of 24. But they have logged some critical seasons of NBA development. And none of them are being counted on the way the Sixers are counting on Simmons and Embiid.