In many ways, the win over the 76ers was personal for the Celtics’ Al Horford

Al Horford hounded 76ers guard Seth Curry during the second half of the Celtics' win.Charles Krupa/Associated Press

When Al Horford turned down the final year of his contract with the Celtics after the 2018-19 season, he believed he left for a better chance at a championship, a situation that would use his skills more to his liking, reduce the wear on his aging body, and provide an opportunity to play beside a top-three center.

About 20 games into the experiment of Horford playing power forward alongside Joel Embiid in Philadelphia, it was scrapped by then-coach Brett Brown. Horford was relegated to a reserve, playing when Embiid was out of the game because the two couldn’t coexist.

While the money was enticing — four years for $109 million, a sum the Celtics scoffed at — Horford was entrenched in a detrimental situation and both sides quickly regretted it.

When the 76ers moved Horford to basketball-contract Siberia — Oklahoma City — in December 2020, it confirmed the situation was an abject failure, and it has taken more than a year for Horford to regain his confidence and effectiveness.

Al Horford and Philadelphia's Joel Embiid battle under the basket in first-half action Wednesday night.Jim Davis/Globe Staff

He coincidentally returned to Boston on the same deal the Celtics wouldn’t consider in the summer of 2019. And Wednesday night’s matchup with the 76ers was indeed personal.

A handful of the 76ers remain from Horford’s days in Philadelphia, and while the coaching staff and management were different, it was still general manager Daryl Morey who traded Horford to the Thunder a few months into the job.

A few minutes into the Celtics’ first meeting with the 76ers this season, it was apparent the mild-mannered Horford was ignited. He scored 7 of Boston’s first 11 points and added a block of a Tyrese Maxey soaring layup attempt.

And on the final play of the Celtics’ razor-thin 88-87 victory, Horford was switched onto former teammate Tobias Harris, and Horford’s defense was so effective, Harris forced a pass to Georges Niang, whose 3-point heave was partially blocked at the buzzer.

Horford let out a roar after the victory was sealed. It was vindication of sorts for the 35-year-old five-time All-Star. He was considered finished in Philadelphia, unable to contribute to a team with championship aspirations, perhaps one of the reasons the 76ers were swept by the Celtics in the Orlando bubble. He heard all the criticism and took it personally.

“I’m very grateful because my faith kept me strong through that time,” Horford said. “It was a very low point for me at the beginning when it all went down, looking at having to go to Oklahoma City with me in my 14th year. But I really looked at it at that point as an opportunity for me to get better, to prove myself and to prove to people what I can do. That year was a difficult year for me in Philly, no question about it.”

Celtics coach Ime Udoka was an assistant on Brown’s staff during the Horford experiment. Horford’s numbers during that season weren’t as disappointing as perhaps the 76ers’ actions reflected. He averaged 11.9 points and 6.8 rebounds with 35 percent shooting from the 3-point line. But he wasn’t that hot-shooting stretch-4 the 76ers wanted, and Embiid’s presence in the paint forced Horford to stand on the perimeter.

Al Horford is in his second stint with the Celtics.Michael Dwyer/Associated Press

“None of us loved the way it went in Philadelphia for him, and the team in general,” Udoka said. “It sounded good. Joel, who missed about 20 games a year, so you had your backup in play that could also play with him. But we just never found our footing with him. I don’t think we used him properly with some of the matchups he had there.

“But when you’ve got a guy like Ben Simmons and Joel Embiid in the post, and in certain areas they’re taking away a lot of his space, so he’s kind of standing around at times. But you saw the flashes last year in Oklahoma City; he was a different player, back to himself.”

Horford was the Celtics’ return for moving Kemba Walker to the Thunder, and Walker already has played his way out of the rotation with the Knicks, whom he signed with after being waived by Oklahoma City. The Celtics couldn’t be more pleased with the Horford they welcomed back, close to the player he was three years ago and able to play his preferred power forward, with Robert Williams at center.

And while Horford has redeemed himself after a couple of hard years, he was admittedly embarrassed and regretful that the Philadelphia experiment didn’t work out.

“No question about it,” Horford said when asked if his reputation took a hit. “But it’s everything on me. It was my decision to leave [Boston], and it was like, how are you going to respond when you’ve been faced with adversity? Being down, being talked down about and those things. I was written off and I’m glad I got another opportunity in a place where I want to be. But no question about it, there’s also a lot of satisfaction being back here and playing at a high level.”