If I'm not mistaken, Amir Johnson and Jonas Jerebko will walk and probably get larger contracts with other teams.
There's a chance KO leaves as well if the C's don't reach an extension with him.
So that leaves Boston with Horford, Zeller and Mickey as their "bigs" next season (After Horford, yuck). Zeller is just a "tradeable contract" at this point who is below-average this season (though his mid-range J's are falling, his defense is atrocious and he can't match up well against opposing bigs most nights)
That's going to be a whole bigger issue going forward. Everyone says "sign Hayward", but if that's the logic.. then rebounding and shot-blocking will get A LOT WORSE, and it would sort of negate the impact of signing Hayward to a max.
Getting size is what will help BOS become contenders to challenge GSW (look at yesterday's game without Crowder + Horford), and Cleveland. Playing small most of the game won't, unless you'd like Tristan Thompson alone to kill us again.