I dislike the entire idea of "core" players for this roster. We don't really have any clear core players at this point, except Horford but he's older so not who people are talking about.
We don't have a "core" in the sense of a championship core who's just waiting on the right compliments, but I would say we have a decent little core on the perimeter in IT, AB, Jae, Smart, and Jerebko.
It was really our bigs rotation that hurt us more than anything else last year, so adding Horford to that core was a great step. KO and Amir are both serviceable.
I wouldn't say any of those 8 guys are untouchable, but they're not the type of guys that have other teams calling Ainge and offering great assets/packages in return. I'd trade any of them in a heart-beat if it's a clear upgrade, but I wouldn't mess with our group of ultra-scrappy players for anything less. A bird in the hand is worth two in the bush, and we already know these guys play well together.
Jae, Smart, and KO especially are guys who we're better off waiting and seeing what they become vs. accepting their current value back in a trade.