I think Humphries can play center, but I think he's ideally a solid rebounding PF who can hit the 15-18 footer.
Regardless of where we slot him, he will take away minutes from Sully and Olynyk. I don't think either are going to become All-Stars next year, but they need minutes to even entice others to want to trade for them.
I think the Celtics, especially after signing AB, need to be very conscious of not spending much more money just to say they "did something" this offseason. Letting Bayless and Humphries walk is a good move, imo.