If things go bad in the playoffs, moving Al's contract won't be easy. Anyway, I would rather try to keep open as many options as possible to be able to make in-season trades up to February 2018.
Offseason plan:
Draft as well as you can. We might get #1 pick, or not. I'm guessing we get the #3 pick, and Fultz and Josh Jackson are out, so pick Ball.
Let Amir go. Zizic gets his spot.
Trade Tyler Zeller and Jordan Mickey to Utah for Joe Johnson and a protected future pick. Johnson gets Gerald Green's spot and becomes an expiring which you could use at the trade deadline.
Sign Anthony Randolph from Real Madrid, he's having his second great season in a row playing in Euroleague and he could be a great bench player for the Celtics. Try to negotiate buyout with Real Madrid, maybe Yabusele could be stashed there
Try to resign Jerebko for reasonable money, if not, let him go elsewhere.
Try to resign James Young for bargain money, if not, sign Nader.
Send one of DJackson/TRozier in a minor trade for a young big (i.e. Demetrius for Noah Vonleh) or future picks.
This would be the roster to start the 2017/2018 season:
IT/Ball/Rozier or Jackson
AB/Smart/Johnson
Crowder/Jaylen/Young or Nader
Horford/Randolph/Jerebko or FA
Zizic/KO/Young Big