Looks like Chicago isn't willing to spend the money to keep Nate. No, he is not perfect, But he has shown great improvement in consistency and maturity. He has also shown he would be a great stand in until Rondo returns (if he returns), and a great backup should Rondo return.
Boston has many unreliable smaller players headed for the roster. Compared to many of them, Nate offers more consistency. He proved it in Chicago this past season. He only earned the minimum last year. $5M (per year - multiple year) would close the deal.