How high can the Celtics rise this season?

One of the more interesting aspects of this edition of the Celtics is that they know they are going to be good, they’re just not sure how good. They know they are capable of greatness, but not sure how great. And they know they have talent, but how much talent?

It’s a season of excitement and uncertainty for the Celtics. The rebuild is officially over. There are high expectations for this team. The Celtics are viewed as the top challengers to the Cleveland Cavaliers in the Eastern Conference, the defending world champions.

At least we think.

With the addition of Al Horford, it will take a handful of games for the Celtics to find their chemistry. But if the preseason was any example, Horford’s impact will be monumental. Horford has blended into the team concept flawlessly, playing the type of unselfish ball he exhibited during nine seasons with the Atlanta Hawks, and he has added quiet leadership and professionalism.

The Celtics invested a maximum contract in Horford, and Horford is invested in Boston. A sparkling career in Atlanta resulted in just one trip to the Eastern Conference finals. So he wanted to be sure that the Celtics had the same sense of urgency that he carried.

“I’m very happy, I just feel that coach [Brad] Stevens is a great coach,” Horford said. “He’s really taking his time to kind of reel me in and get me up to speed with everything. And it just reassures my decision, I guess.

“I didn’t expect guys like Terry Rozier to have so much improvement from when we saw him in the playoffs until now. He’s a totally changed player, much, much more confident. That was one of my questions [before I signed], I didn’t know who was going to be backup point guard. Marcus Smart has impressed me even more.”

Horford was intrigued by the Celtics’ roster. From an outsider’s perspective, he saw a growing team that was not close to its peak and needed a veteran presence. The Celtics, without really realizing it, were selling themselves to free agents. They nabbed Horford and nearly captured Kevin Durant.

“I was like, it would be great to play with those guys,” Horford said. “And we can build out from there. All of that other stuff is really up to [president of basketball operations Danny Ainge]. They will determine those things. But I enjoy playing with this group of guys here and just getting to know them.”

Ainge has loaded his roster with young, hungry players. The past two seasons of first-round playoff failure has motivated the core players to become even better. The losing made this team hungrier, and the younger players have embraced leadership roles.

Horford raved about the fiery attitude of Smart. Jae Crowder acknowledged he was not near 100 percent healthy during the playoffs and felt slighted by being left off the NBA’s All-Defensive team.

“I don’t want to lose a game and I want to be prepared as possible,” Crowder said. “I’m going to do my part, for sure. I’m locked in. I’m ready to go. I’m fresh. I’m at a good area in my life and I just want to play basketball and work on our goals and win games.”

The Celtics are expected to rise in the East in part because other teams have regressed. Miami lost Dwyane Wade and Chris Bosh. Atlanta lost Horford and replaced him with the mercurial Dwight Howard. Charlotte lost Al Jefferson and Jeremy Lin.

Toronto’s lone free agent acquisition, Jared Sullinger, the former Celtic, is out indefinitely after having surgery on his left foot. Detroit will play without former BC standout Reggie Jackson at point guard for the first two months.

Smart may miss Wednesday’s opener against the Nets, and Kelly Olynyk should return from shoulder surgery in mid-November, but the Celtics have been mostly healthy this offseason. And the fact that Horford, rookie Jaylen Brown, and the well-traveled Gerald Green have blended in well should give the team a jump-start this season.

“We’ve got more continuity back and ever since the All-Star break of ’15 we’ve had a pretty stable group,” Stevens said. “Having people understand what you’re trying to accomplish, how practice works, how it flows, what we’re looking for in film session, all that stuff helps. That certainly is expedited when you’re talking about 30-year-olds like Al or Gerald Green.

“We’d better be a blue-collar team that competes at a high level. There’s not a lot of room between finishing 10th and second last year in the East, and ultimately if you want to be among those considered the best, there’s a lot of hard work ahead.”

Kind of overshadowed this training camp has been All-Star guard Isaiah Thomas, who began last season as the sixth man and emerged as a cornerstone. Thomas said he has worked on his conditioning and nutrition for the purpose of increasing his endurance.

The Celtics relied heavily on the 3-point shot last season, but Thomas converted only 36 percent of his attempts. If the Celtics are to take that next step, they’ll need to improve their 3-point shooting. Avery Bradley became more of a long-range weapon last season and wants to emerge as an elite shooter.

“My goal is every open shot I take, I want to be able to knock down,” he said. “I want to shoot over 40 percent from the 3. And I’m able to make a lot more plays for my teammates because I’m confident in my ballhandling and play-making and my decision-making. I feel a lot more comfortable out there.”

The addition of Horford and the overall improvement of the returning players have created great optimism in Boston. But no one, not even the players, knows what the Celtics’ ceiling is.

The Cavaliers have proven themselves. They have the best player in the game in LeBron James and a standout offensive point guard in Kyrie Irving. Cleveland lost Matthew Dellavedova to Milwaukee, but added Mike Dunleavy and Chris Andersen. They are the prohibitive favorites in the East, and James is the leader, prepared to prove that last season’s championship was no fluke.

This Celtics team is more prepared and experienced than last season’s, but even management acknowledges that championship piece is missing. This team will be entertaining and have a chance to win on a nightly basis for the first time in Stevens’s tenure. But there remains a sense of uncertainty. How will all of this work out?

That question will hover over this team until it goes through the season together. The journey won’t be easy.

“I almost think there’s too much noise about the team and we actually have to do something,” co-owner Wyc Grousbeck said. “I also think we’re a step below where we want to be. So I don’t think is ‘the’ team, but I do think it’s going to be a good team to watch and we’ll be hopeful that we keep making steps forward.”

Gary Washburn can be reached at gwashburn@globe.com. Follow him on Twitter @GwashburnGlobe.