Sully will start. He has the potential to be the best player in the frontcourt. Also, he showed his value in the playoffs when he came in and played tough down low when KO and Zeller just couldn't. Those two still can't. They just don't have the ability to be tough down low ayers like Amir and Sully.
So I say it's Amir and Sully starting with Zeller and Lee coming off the bench with KO getting minutes when someone gets hurt or Stevens needs a stretch four to hit shots because the team has fallen behind big and needs offense. KO will definitely have a role but his role will be in the most flux and depend on the most variables whereas Amir, Sully starting and Zeller, Lee coming of the bench will be sure things.
Now if Sully shows he can't play 30 MPG or starts adding weight on during the season, that changes everything. Then we could see Zeller and Amir starting with Lee, KO and Sully rotating off the bench depending upon need and matchups.
No matter what the combo, if all five players stay healthy, expect at least one to be traded early in the season. Ainge won't wait for the trading deadline to deal one. He will get trades done early to get the best value.
I think Mickey is definitely someone Ainge wants on this team in the future, more so than many of the five guys in from of him. So I expect a trade early so time can be given to Mickey as th fifth forward and have him start his development.