I think we have our center, he is Al Horford. In my humble opinion, he can't play the 4, specially in the modern NBA. I think he can't defend well in the perimeter and he is offering solid rim protection at 2.6 bpg.
I think the front office needs to look for a starting PF, which the Celtics are lacking. I like Olynyk and Jerebko, but neither is starting material.