I doubt the Celtics want to give up assets now for Drummond. The whole Celtics rebuild feels like they are gearing up to be dominant in the East once Lebron's time has passed in 4 years or so.
However they have cap space, and lots of it, especially after next season.
I've heard many speculate, some informed some not, that Hayward could very well opt out of Utah this summer and sign with the Celtics.
Also Blake Griffin can opt out and become an unrestricted free agent after this season. Things have been pretty stagnant with the Clippers since CP3, Jordan and Griffin were united. You'd have to think Blake is considering his options, although it's hard to imagine anyway getting up and leaving the city of Los Angeles for Boston.
Also, if the Celtics do indeed end up with the 1 or 2 pick in the draft in June, and take Ball or Fultz, I think there's a very real possibility Isaiah is traded.
His contract will be up after 2017/18, he's on a very team friendly contract right now, and he will be demanding max money (as he's already pretty much stated after Horford was signed). I don't think he's worth max money. Love the guy and his offensive ability, but he's also 5'9" and a defensive liability, don't think that type of player deserves 5 years, $150 mil or whatever it is. If they can convince him to sign for far less, then that's a different story.
Rather have Ball/Fultz for the future rather than a 30 year old 5'9" point guard who is making $30 mil a year. Plus they could surely get a tidy return for IT.