Here's a related question: What's Draymond Green's best position? I'd say center, because the Warriors' best lineup is one that pairs him with Kevin Durant and Iguodala. He's the default "biggest" guy on the court (yes, I know Durant is taller).
Similarly, the Celtics' best lineup is one with Horford as your biggest player. If you want to call that a center, sure. That lineup should also have other guys that can do pretty much everything on offense, and guard pretty much everybody on defense. That's how you win in the NBA these days.
As for a starting lineup: You should pair Horford with an Amir Johnson or Aron Baynes type, just like Golden State started Bogut and Pachulia the past couple years, even though they didn't typically finish games. Gives you some toughness to start the first and third quarters, and it saves Horford over the course of 82 games.