You don't need a league leading shot blocking/rim protecter, but you need someone that can detour teams from attacking the paint/rim. I would say every team that prepares for the Celtics are begged to take it to the basket. How many times have opposing bigs/centers had career games against the C's this year?
Talking just defense: Lets face it, Zeller has flashes, but is basically soft and not getting the minutes he should, he's more of an offensive guy anyway. Sully's to short, and can't leap at all and really not a center, in fact I'm not sure Sully can defend even forwards. Kelly Olynyk is 7' with short arms and is "small forward minded" he's happy on the perimeter, "bless him" he tries, but he is not a defender at all.
Yes, the Celtics need a tall, long, athletic Larry Sanders type ( not saying it's LS) to defend the paint, not necessarily score much. Then see which remaining PF can defend the other teams PF's and be of value of offense.