"I won't be able to go tonight," the center said shortly before his Philadelphia 76ers faced the Celtics. "Went out there on the floor, [and] just not where I need to be."
Horford, who was listed as questionable by the Sixers on Thursday morning, went out to the court for his usual pregame workout and met with the team's training staff afterward. It was determined then that he wouldn't be able to play.
"It's a special place to play at," Horford said. "I'm definitely disappointed that I won't be able to play tonight."
Horford joined the Celtics as a free agent three years ago, after spending the first nine seasons of his career with the Atlanta Hawks. He helped them to back-to-back Eastern Conference finals in his first two seasons before Boston lost to the Milwaukee Bucks in the 2019 conference semifinals.
It came as a surprise in June when Horford opted out of the final year of his contract with Boston and then joined the Celtics' archrivals this summer. But he said the decision to leave the Celtics was a much more difficult one than the one he made to join them.
"For me, it was a big leap of faith coming here," Horford said, "leaving my stability and everything that I had in Atlanta and then I didn't realize that what I was building here was something that I felt like was very special with this group and everything.
"It was definitely tougher when I had to make the decision to leave [Boston], but ultimately I understood that was the right decision for me to do."
Horford said the reason it was the right decision for him was because it gave him the opportunity to play alongside Sixers superstar center Joel Embiid. During his time in Boston, Horford was considered one of the league's best defenders against Embiid -- and, frankly, one of the few who could slow him down at all.
Now, though, Horford -- who turned 33 in June -- doesn't have to take the constant pounding of guarding centers that he did in Boston, and he and Embiid being on the same team allows each of them to be more prudent about managing injuries like the ones he is now.
"Really just looking at that opportunity," Horford said. "I know that I really felt like the opportunity to go over there and play alongside a guy like Joel and Ben [Simmons] was something that was going to be hard for me to pass. To have that opportunity to, I feel like, win now -- that was one of the things that really drove me."