Give IT some room to take the high road, and except an offer of less money than he "should" to win a ring in Boston.
Let him have the room to say, "I'm a Boston Celtic first last and always, I want ring in Boston. I'll do what ever the organization feels is best for the team. It's not alway about dollars for me." Hey, you never know, maybe he and his agent don't get the interest they think they will.
If he leaves room open to come off the bench, as the NBA's best 6th man...give it to him. Because with him coming off the bench, playing behind a suitable two way PG, he'll actually lead you into a finals appearance.
The guy is an offensive weapon, used properly.