The best/most realistic trade for the Celtics is:
Bradley, Brown, Zeller, and Grizzlies Pick for Butler.
They get established starting talent in Bradley, young and upcoming talent in Brown, an expiring deal in Zeller, and a pick with the opportunity to be a lottery pick. That's everything you want in a trade for a star and in return we get to keep our depth on the roster, both our Brooklyn Picks which are another way for us to add another star, and add a player that is one of the best 2 way players in the league.