I am also pessimistic about Shaq's return, but I like him with the starters. I think he can help establish Rondo's game early by converting easy attempts off point guard penetration. This gets Rondo in attack mode and well we all know the adage, "as rondo goes so goes the Celtics."
On the flip side JO brings height, rebounding and shot blocking to the second unit. Also, for this next round match up, he can also step out and guard Bosh or Z. This would otherwise be BBD and I don't think he matches up well with either of those assignments.
However, you gotta put your best 5 out there to close, so if I was Doc, JO is closing at C.