Neither team does that.
The Kings love Fox and he is cost-controlled for some years.
The Celtics need Smart for this second unit and don't need another PG having Rozier as second unit and Larking as deep bench. Also, if we trade young people should be for consolidate players, given the amount of rookies on the roster. Finally, some people may argue Yabusele is an intriguing prospect with the potential to be an important player down the road (not sold on this, but understandable).