While the Sixers will be focusing on finding a coach to replace Brett Brown, who was fired on Monday, eventually the organization must build a team for the 2020-2021 season.

In putting it together, the Sixers may want to look at the team that just swept them, the Boston Celtics.

That’s not to suggest the Sixers should suddenly go to a small-ball lineup like the undersize Celtics, whose starting center was 6-foot-8 Daniel Theis.

One reason the Celtics eliminated the Sixers so quickly involved an intangible: toughness.

That is not to say the Sixers don’t possess toughness. Tobias Harris’ return in the fourth quarter of the 110-106 elimination loss Sunday was courageous.

With 2 minutes, 40 seconds left in the third quarter, Harris was undercut while in the air defending on a play. His face smashed hard against the floor.

After being evaluated for a concussion, he returned with 5:21 left.

That was true toughness, but throughout the series, the Celtics showed more mental and physical toughness by a wide margin.

Nobody demonstrated that more than guards Marcus Smart and Kemba Walker. The 6-foot-3 Smart and the 6-foot Walker were Boston’s leading rebounders in the 102-94 win over the Sixers in Game 3 on Friday. Both had eight rebounds despite the Sixers’ height advantage.

Smart is one of the toughest players in the NBA. He backs down to nobody.

Smart’s flaw is that he is not a good shooter. In fact, during the series, his effective field goal percentage (.367) was second-lowest among players who averaged at least 10 minutes. (Alec Burks’ was .357). Nobody would want Smart to take a big shot, but nobody would want him off the court in a critical time either.

“He’s willing to take on any challenge and willing to do anything it takes to win a basketball game,” Walker said of Smart after that Game 3 win. “He doesn’t care, you just need a guy like that.”

For the series, Smart was a plus-24.

His ability to guard any player of any size is also impressive. According to NBA.com stats, Smart had at least 2 minutes, 30 seconds of matchup defensive minutes against seven Sixers. The most was against the 6-8 Harris, whom he guarded for 15:52. Smart even had 2:30 against Joel Embiid, who did not attempt a field goal in that time.

“He is our glue guy,” Walker said.

Walker also displays great toughness. He has been battling a knee injury since entering the NBA restart. On Sunday, he was under the weather, according to coach Brad Stevens. Walker missed the pregame film session while seeing a doctor.

All he did was score 32 points in the clinching win.

“I was a little bit worried about him from the standpoint of how he felt, but he is just a warrior, a competitor,” Stevens said afterward.

Theis continued to make Embiid work hard, despite the size disparity. Jaylen Brown also constantly fought against bigger players, and the list goes on.

In Game 2, after the Celtics’ 109-101 opening win, the Sixers came out on fire, took a 25-11 lead, and looked intent on evening the series.

The Celtics barely blinked, led, 65-57, by halftime, and routed the Sixers, 128-101.

In Game 3, the Sixers led, 94-92, when Boston scored the game’s final 10 points. Boston’s best player, Jayson Tatum, picked up his third foul late in the first quarter. He didn’t play the entire second period and the teams played to a 25-25 standstill.

The Celtics didn’t allow any adversity to bother them.

The Sixers were without two-time All-Star Ben Simmons, and no doubt that was a big blow. But Boston played without Gordon Hayward for the final three games, missing its second-best three-point shooter in the regular season.

That’s not to suggest that Hayward offers the versatility on both ends of the court that Simmons does, but he did average 17.5 points, and a big chunk of the offense left with his departure.

The Celtics were tough enough to focus on the task at hand, not worrying about what they didn’t have. So when the Sixers prepare for 2020-2021, they should add some toughness.

Not an easy task, but it’s necessary.