Al Horford left the Celtics to sign with Philadelphia, a move that didn’t work out for either side. Horford has returned to Boston, after a stop with Oklahoma City, and is thriving. Michael Dwyer/Associated Press

BOSTON — Al Horford will be remembered as one of the 76ers’ worst free-agent signings.

On July 10, 2019, the post player signed a four-year, $97 million contract with the Sixers after opting out of the final year of his contract with the Boston Celtics.

While the contract was lucrative, the move had a negative impact on Horford’s career. Not a good fit with Joel Embiid and Ben Simmons, he was demoted to backup center.

But he’s back with the Celtics, and playing at a high level. He even had a starring role in Boston’s 88-87 victory over the Sixers on Wednesday night at TD Garden.

“No question about it,” Horford said when asked if his reputation took a hit while playing for the Sixers. “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’re 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 of being back here and playing at high level.”

Horford had a solid first quarter against the Sixers, scoring seven points in the first four minutes. He added a third-quarter 3-pointer en route to finishing with 10 points on 4-of-6 shooting, including making 2 of 3 three-pointers. The 35-year-old also finished with eight rebounds and five blocks in his first game against the Sixers since being traded to the Oklahoma City Thunder on Dec. 8, 2020.

As solid as he looked on Wednesday only magnifies the huge miscalculation the Sixers made signing him in 2019. The team basically admitted that by demoting Horford to the bench before the Feb. 11, 2020, home game with the Los Angeles Clippers. He returned to the starting lineup three games later only because Simmons was sidelined with a pinched nerve in his lower back.

Horford, as planned, was able to step in for Embiid, who missed 22 games that season. But even that was a bad justification, considering the Sixers basically paid $28 million to a backup center.

They all saw it is a mistake.

It became obvious that Horford was out of position as the Sixers starting power forward. At center, he was in his comfort zone, quarterbacking the gym.

“None of us loved the way it went in Philadelphia for him and the team in general,” said Celtics Coach Ime Udoka, who was a Sixers assistant that season. “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 we had in there.”

Horford basically stood around at times while Simmons and Embiid ate up all the spacing.

“But you saw the flashes last year in Oklahoma City,” Udoka said. “He was a different player; back to himself.”

Horford’s scoring average of 11.9 points as a Sixer during the 2019-20 season was his lowest since averaging 11.5 points as a second-year player in 2008-09. Last season in OKC, he averaged 14.2 points while only playing 28 games before being a healthy scratch for the rest of the season.

Horford is now averaging 12.4 points in Boston and producing 1.8 blocks per game, which is good for eighth in the league. He’s had five or more blocks in three games.

“I’m very grateful, because my faith kept me strong through that time,” Horford said of his tenure with the Sixers. “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 Horford then looked at it as an opportunity to get better and prove to himself and people what he could do.

“That year was a difficult year for me in Philly,” he said, “no question about it.”

THE CELTICS RETURN to full health lasted just one game after the team announced Jaylen Brown would be sidelined Friday against Utah for injury management.

Coach Ime Udoka said Brown’s absence at the beginning of the team’s road trip stems from a Brown feeling some tightness on Wednesday night against Philadelphia in the hamstring that caused him to miss eight games last month.

“He had a little tightness late in the game the other night, and we listed him questionable, day-to-day, and that’s how it’s going to be going forward off this injury,” Udoka said at Friday’s shoot around.

Brown has played in each of Boston’s last five games but has been on a minutes limit as the team attempts to ease him back in after the injury. However, Brown did not look to be moving at close to full strength on Wednesday night nearly a week after his return to the court and that weakness was noticeable to Boston’s bench.

“He just got tight,” Udoka said.

Brown played the most minutes since returning on Wednesday night (33) but he also struggled the most since his return to the court, scoring just nine points on 3-of-11 shooting.

DANNY AINGE almost always finds himself in the NBA front office rumor mill. Friday was no different.

Following the Portland Trail Blazers axing of Neil Olshey for violating the franchise’s code of conduct, Ainge’s name was among the initial replacement candidates.

It makes sense. Ainge has a track record of success in a front office role in the NBA stemming from his 19-year tenure with the Boston Celtics and has local ties. Ainge was born and raised in the state of Oregon and played two seasons with the Blazers.

Ainge, 62, stepped down from his position with the Celtics after last season. It isn’t clear what he’s exactly looking for at this stage in his career (he hasn’t formally interviewed for any front office jobs since departing the Celtics) but the sense has always been he would return to a team front office in some capacity.