Durant is eligible to sign for $26.6mm next year, whether with OKC or another team.
Currently, BOS has $51.85mm in active contracts (including $17.00mm for Johnson and Jerebko) and $25.40mm in active cap holds. That leaves $16.75mm in cap room. For BOS to fit Durant into their roster, they will have to do the following:
Easy moves:
1. Release the cap holds on Turner and Datome ($4.45mm and $2.28mm, respectively).
2. Release Holland ($0.87mm).
That would give BOS $24.35mm in cap room, without cutting into muscle, let alone bone.
Renouncing the cap holds for Sullinger ($5.67mm) or Zeller ($6.54mm), or failing to pick up the July 3rd options of Johnson ($12.00mm) or Jerebko ($5.00mm), would clear the remaining room for Durant.
Renouncing all of these contracts would give BOS a total of $53.56mm in cap room, which would be just enough to sign both Durant and Horford to 1+1 max contracts (whereby both players would have player options for 2017-18, or could tear up those contracts and sign higher max contracts next summer). Keep in mind that, because the Johnson/Jerebko options are July 3rd, Durant and/or Horford would either need to make up his and/or their decision by then or the Celtics might be cutting players they might need if one or neither sign with Boston.
Resulting roster (assuming both Durant and Horford sign, Zizic remains in the US but Yabusele goes back to France for one more year):
C - Horford, Mickey, Zizic
PF - Crowder, Olynyk, Bentil
SF - Durant, Brown, Young
SG - Bradley, Smart, Hunter
PG - Thomas, Rozier, Jackson
+rights to Yabusele