While I like the M. Miller idea as the type of player he is, there is a much cheaper version available. Gordon Hayward of Butler in the draft. 6'9", 83% FT and can score from anywhere.
Draft Hayward, save money for replacements for Sheed if he retires and try and keep T. Allen and possibly Ray Allen if he is cheap.
Celtics need youth to go along with big Perk, Rondo and Big Baby. Hayward would be cheap, same skill set as Miller and available at the #18th pick.