He was a key piece, not a core piece. Big difference. If he was a core piece, they wouldn't be trying to dump him and use assets to do it.
And this last season, Lee was DNP-ed for half of it and played 18 mpg in the games he did play. Green was a rotation player previous to this season, he became a core piece this year, which is precisely what made Lee expendable and not a core piece.
Sounds more like splitting hairs to me. Hood averaged 21 mpg, Im pretty sure everyone refers to him as a core piece.
The point was they added Igoudala before the whole Lee not playing many minutes thing, while he was still considered part of the "core" (which I believe includes Green).