Obviously this may depend on what salary comes back if Brogdon is dealt, but I don't really think the Celtics want to go over the tax just to keep Grant. I'm beginning to think it might be the best move to try and trade him for a late first/early second, which the Pacers could provide. Take a step back, draft two guys you think could be part of the rotation (or one ready to play + one project) and enjoy their production for a cheap salary for 4 years, maybe more.
EDIT: I don't think the Pacers have any reason to deal Jackson since he has two years left.