He's no rim protected but his d isn't terrible either. He is slow big has great handle and is s good passer. Nothing wrong with being a backup either. I think at least he should too out as a perennial 6 man candidate at least. Not saying that's worth 20 mil but there's always some team ..
Not a bad summary.
His team d is excellent, and undervalued, in my opinion, on this forum. He's a net plus, especially in a bench role where his exposure can be limited.
Brad Stevens lumps "bigs" together, but in practice Kelly is almost exclusively a center, where his defensive limitations are minimized, and where he pulls a big big away from the rim on offense.
I think he's a 7th or 8th man, "first big off the bench" type (not to minimize his uniqueness, which is an edge). He's definitely not the classic 6th-man, i.e., a versatile scorer; then again, the Celtics pioneered the innovative use of guys like Paul Silas in the role.
If things go well for Boston, I look for Jaylen Brown to emerge as 6th-man after the trade deadline.