I don't think it makes any sense to trade a prospect like Fultz for anything short of the Greek Freak or Curry.
For example:
Jae, Rozier and BK 18 for Porzingis and Kuzminskas
That is giving Porzingis up for like $0.75 on the dollar with the chance of making $2.00 on the $1.00.  It gives the Celtics a rebounding big who can shoot the 3 with long term potential, a decent wing and $1.5 million extra in cap space.
Then renounce/waive/don't resign: Amir, Zeller, Jerebko, Young, D. Jackson, Mickey, Green for $30.5 million.  Sign Hayward.
Then we have:
IT / Fultz
Bradley / Smart
Hayward / Brown
Horford / Yabu
Porizingis / Zizic
And I'm sure we could get some vet minimums in there after that.  Title contender next year and a young core of Fultz / Smart / Brown / Zizic that will contend down the line, too.
Why give up Fultz?  I don't get it.  We have so much flexibility to add something without giving up a player with the ceiling like that.