This current Celtic squad adv is defensive execution/trap/team .D
And offensively, running game (especially having the bigs run in the transition) , spacing (again bigs pulling out their player), creating space to cut to basket or drive
Sully while good at rebounding (1 on 1) is not good at the things above. Tries 1 on 1 play at the offensive end and suceeds here and there but this is against CBS system (considering current roster)
He can't shoot on a consistent basis. No existing post nor pnr game. OK mid range game that he barely opts to take advantage of