Even if Sully's back next year--which I'm certainly not convinced he will be--there should be room to add a young player to the big man rotation.
My preference would be to let Amir and Sully go, and re-sign Zeller to a cheap multi-year deal.
Zeller on a cheap deal (because the Celts have torpedoed his value this season by screwing with his role -- thanks to the awesome David Lee trade) will be a much better trade asset than Amir Johnson on an expiring, especially when you factor in the likelihood of Amir being injured for a significant chunk of next season.
Olynyk, Zeller, Mickey, Jerebko and then maybe a rookie and / or a free agent signing becomes your big man rotation. I could take or leave Jerebko, but he can play multiple positions / roles and he's cheap.
The Celts don't really need to have too many big men on the team, because the team is at its best going small. On a lot of nights they won't be able to have more than one of them on the floor for any extended period of time anyway.
I'd much rather keep Amir than re-sign Zeller. Tyler's a decent athlete who runs well and can hit a jumper, but he's a poor defender who isn't skilled enough offensively to make up for it.
I don't hate him as a bench guy, but personally I don't think there's any amount of minutes that are going to convince anyone Tyler's a high value asset.
I don't know what Amir Johnson's trade value is, but I know he has value on the court. I would like to see him stick around for that on court value (unless of course we can upgrade his position--then I'd be fine with letting him find another home).