CAMDEN, N.J. — As much as Al Horford tries to fit in here, like he’s just one of the guys, like he’s a true 76er, he can’t just yet.

It was less than two years ago that Horford and Jayson Tatum were celebrating on the Wells Fargo Center floor, the rookie grabbing the veteran by the shoulder after Horford scored the clinching bucket for the Celtics in Game 3 of the Eastern Conference semifinals.

The bond seemed as if it would last for years. Tatum and Horford, young and old, emerging All-Star and former All-Star. But it was over a little more than a year later. Horford stunned the Celtics by not only opting out of the final year of his contract at $30 million, but then spurning their extension offers a full two weeks before free agency even began.

Horford instead signed a four-year, $109 million contract with the rival 76ers, with $97 million guaranteed. With Jaylen Brown, who just signed a four-year extension for $115 million, Tatum with his big payday coming, and Gordon Hayward on the books, plus acquisition Kemba Walker, the Celtics believed they just couldn’t afford an aging Horford earning $25-plus million in the final year of a new deal.

It was amicable parting. Horford, 33, has had nothing but respectful things to say about his three years in Boston. But Wednesday night’s season opener against the Celtics will be awkward for him. He fully expected to spend more years with the Celtics, more years tutoring and mentoring Tatum and Brown.

“You are facing everything you were a part of before,” Horford said Tuesday following practice. “It’s going to be just different, playing them. It’s a great opportunity for me. I’m put in a weird position here. It’s best for me to just go out and do it and cement myself here.”

But the NBA is a business, and Horford thought less like a player and more like a businessman in his decision to sign with the 76ers. He knew he would play center considerably less, meaning less pounding on his body. He would receive a chance to play for a championship on a wildly gifted team, and he would do it with plenty of money in his pocket.

Al Horford spent the last few years with Brad Stevens and the Celtics before leaving for Philly. Jim Davis/Globe Staff

The past three years aren’t lost on him, however. He developed into a fan favorite as the Celtics faithful took their sweet time appreciating his game, and he turned into Dependable Al, a staple in an organization that was ever-changing and undergoing growing pains.

He recalls that Game 3 against the 76ers in 2018. When the Celtics took a 3-0 lead over the favored 76ers, when the Kyrie Irving-less club was in the midst of an improbable playoff run, Horford was surrounded by a bunch of youngsters who had no idea of the magnitude of their accomplishments.

“It is very surreal,” Horford said. “I would have never imagined being in this position. For me it’s a great challenge ahead, being here and trying to accomplish something special here.”

The situation in Philadelphia is optimal for Horford. He gets to play with one of the league’s top centers in Joel Embiid. Ben Simmons could emerge as an MVP candidate. Josh Richardson is one of the league’s more underrated players. Tobias Harris just signed a maximum contract extension and has All-Star potential.

Horford just needs to be Horford, defending, rebounding, and knocking down the occasional 3-pointer.

Al Horford is ready to open his first season with the Sixers this week. Chris Szagola/FR170982 AP via AP

“It’s good to have expectations, I believe,” he said. “You want to come out and you want to compete. You want to do special things. People see that we’re capable of them, but we have to go out and do them. It’s one thing that I learned [from Boston] is because you’re expected doesn’t mean you’re going to get it done.”

Last season will serve as a painful lesson for those Celtics. No one came away unscathed, not Irving, not Tatum, not Brad Stevens, and not even Horford.

While Horford didn’t have much involvement in the locker room chaos, his quiet demeanor and lead-by-example style didn’t influence his teammates as he would have liked. Last season was frustrating and demoralizing for the players.

The question is, if the Celtics had fared better, perhaps reached the Eastern Conference finals, would Horford have been more inclined to stay? But both sides may have felt it was time to part. Horford joins a team where the young core has been together for several years, coming off a Game 7 loss in the conference semifinals.

After their time together in Boston, both Al Horford and Kyrie Irving have taken their games elsewhere. Barry Chin/Globe Staff

The Celtics have a retooled team, trying to regain their swagger and respect around the NBA. Horford’s three years in Boston were successful but ended painfully. He’ll take those lessons and bring them to his new team, as awkward as wearing the 76ers uniform may be right now.