Assuming no trades, these are guaranteed:
Al Horford
Amir Johnson
Avery Bradley
Tyler Zeller
Isaiah Thomas
Jae Crowder
Jonas Jerebko
Jaylen Brown
Marcus Smart
Kelly Olynyk
Terry Rozier
Jordan Mickey
Semi-guaranteed:
Gerald Green
Battling final 2 spots:
James Young
Demetrius Jackson
R.J. Hunter
Ben Bentil
These final spots all have similar contracts, but not the same.
For example, Ainge would have to make a decision on the team option of Hunter and/or Young in in October 31st, and decision I don't think Ainge would want to make when he's also playing the cap space game next season.
Considering an assumption that there's low confidence on those two, I very much doubt Ainge would pick their options, unless he visualizes an easy exit strategy. That in turn means these two players would become unrestricted free-agents next Summer. So little value.
So, with Young and Hunter he either needs to trade them off or let them go for nothing. Of course, if they somehow become studs then we have them for cheap still (don't think anyone is betting on this).
On the other hand, Bentil and Jackson have long contracts full of non-guarantees. So much easier to manage. This also gives us contracts to trade through the Draft and Free-Agency. They also bring depth to our ball-handlers and some toughness for our big depth.
I don't know where to go. I think Jackson is a fairly easy choice just because we really could use some more ball-handling depth.
James Young is still, well, very young... I wouldn't be opposed to keeping rolling the dice with him.
Hunter has shown flashes of decent basketball IQ, but I don't think he's going to make it, plus he's older. The X-Factor here is the Stevens connection, I think that might play some in this.
Between Hunter and Young, I'd keep Young easily even as he looks quite trashy so far.
I wouldn't be opposed to Bentil over both just because of reasons mentioned above.
I put Green in semi-guaranteed simply because the other players are simply more valuable assets wise. But I think we have too many of that as it is, so we'll end up taking the most prepared player regardless (which is him).