Summary
When Al Horford left Boston in 2019 to sign a 4-year, $109 million contract with Philadelphia, it looked like the 76ers were going to have an elite defense. They had a ton of size, two great interior defenders in Horford and Joel Embiid, and two really good perimeter defenders in Josh Richardson and Ben Simmons. However the 76ers only had the 7th best defense in the league that year, and completely flamed out in the first round after Ben Simmons got injured.

The 76ers cut their losses on the Horford and Embiid experiment and traded Horford to Oklahoma City in 2020, where he played well. The Thunder were competitive for the early parts of the season, and Horford was a big part of that, evidenced by how badly the Thunder played once he and Shai Gilgeous-Alexander were shut down for the latter parts of the season. Horford was then traded to Boston, where returned to where he had some real success from 2016-2019.

Horford had a renaissance in Boston, including looking like an All-Star caliber player at times int he playoffs. He is a skilled big who can operate out of the high post and facilitate some offense with his passing skills as a release valve and secondary playmaker. He can stretch the floor on offense simply by spotting up or operating as a pick-and-pop big. His finishing numbers at the rim have generally decreased in recent years, but increased back to above league average in '21-'22, which helps alleviate some worries about how his age might affect that part of his game.

On defense, Horford is not the quickest guy laterally, so he's not great defending on the perimeter, but he's a really smart defender and overall knows where to be, which is often more important than having quickness and athleticism. It's also not like he's a turnstyle out there, as he can survive and will hold up in most scenarios when being asked to defend in isolation.

At age 36, Horford may not be the All-Star level player he was in his prime, but I he's still a starting-level center, even if that just lasts for a couple more years.
 
Cap Considerations
Horford will be an unrestricted free agent with a $39.8 million cap hold and full Bird Rights, meaning the Celtics will have no restrictions on re-signing him. Since the Celtics will most likely be operating over the cap, and because his cap hold is greater than his expected starting salary, his cap hold is not really relevant.

Since Horford's starting salary will most likely be above the $11.4 million Nontaxpayer Mid-Level Exception, only teams with cap space will be able to sign him. If Horford and an above-the-cap team have mutual interest, a sign-and-trade could be worked out.