I really like Robert Williams. He's a 72% shooter who is averaging 1.2 blocks in 11 minutes for his two-year career. That's 3.6 blocks, 11 points, and 10 boards per 33 minutes.
I'd trade Drummond for Hayward and Williams. I'm not crazy about the #14 in the weakest draft in seven years, especially when we'll already have two rookies in Windler and the #5 pick.
The advantage for the Cavs is after this year they'll have Williams, who will make $2 million this year and the Cavs would control him for four more years. The attraction for the Celtics is they have little use for Heyward (he only played in 5 of their 17 playoff games) but Drummond would be a big upgrade. With him and Theis they wouldn't need Williams.
With more playing time than he got in Boston Williams could develop into a pretty nice Tristan Thompson type player. Except he is a much, much better shot blocker than Thompson (0.9 blocks in 30 minutes last year).