The Celtics are weakest up front at the 4 and 5. If they are to get better, they must upgrade at those positions. Players like Zeller, Sullinger and Olynyk are expendable. Even Johnson is expendable at his salary option. The guys I would hold onto are Mickey and Jerebko. Possibly Johnson.
They need to retool up front with somebody who can score and somebody who can rebound and block shots--protect the rim.
If they have to re-sign Sullinger and keep Olynyk, it will mean to me they haven't upgraded enough to get to the next level.