He's a very good player that fits our system well, even if he's not a transcendent player.
So the next question is: Was it the right move to pay so much money to just a "very good player who fits our system well"? Absolutely, and here's why.
Under the new CBA we're about to see some of the best players in the league sign super-huge contracts under the "designated player exception" clause. For instance, Demarcus Cousins this week made news by hinting that he plans to stay in Sacramento, even though that franchise is mediocre at best with little chance to win a title in the coming years. Why? Because under the new CBA the Kings can offer him $207m over five years ($41.4m/year).
It's going to be really, really hard for the Celtics to sign away one of these transcendent talents because the incentive for those players to stay with their original teams is so strong. It also makes it harder to trade for one. Even if the Pacers are a mess, would they trade away Paul George, their franchise cornerstone, knowing that they can offer him a boatload to stay once his contract is up?
So what the Celtics have done is pay a really good, team-oriented glue guy (but not a superstar) $26-30m over the next four years. In the meantime they will have three lottery picks, including the one they just took in the last draft, Jaylen Brown. All of these players will be super cheap, in comparison to what other good players in the league will be paid (JB will make between $4.7m and $8.6m) over the course of his 4-year deal.) One or more might even evolve into a superstar player.
Horford essentially allows the team to compete at a high level now, while giving the option to either trade your draft capital for a stud to put you over the top, or wait over the next 2-4 years for one of the kids you draft to turn into that stud.
Look beyond this season. We should be glad Horford is here. Even if he's not Tim Duncan in his prime.