The Celtics have a great back court with players such as all-star Isiah Thomas and All Defensive First Team Avery Bradley who makes a good argument for the best perimeter defender in the current league. They're missing a solid center and Okafor is young and proven with room to improve.
The Celtics went far this year in the playoffs with Isiah Thomas and Avery Bradley. Okafor has made immature mistakes last season and could make similar mistakes on the Celtics which could make the Celtics look bad. Okafor is not the right choice to have right now.
I see your point, however the Celtics are mainly missing center right now. I understand that Okafor was immature his first season with things such as the fight outside the bar and in game problems. Still, Okafor is better and younger than any big man the Celtics have and instead of saving there 3rd pick and not trading it for an unproven rookie center they could trade for Okafor who still has a lot of potential immature or not. He is developing into an elite inside scorer. He could easily work more on rebounding and mid range shooting and become a young star in Boston. The environment is perfect for him to grow and mature his game.