From ESPN

The Celtics and the Mavericks have an agreement in
principle to send the All-Star point guard to Dallas for Brandan Wright, Jae
Crowder, Jameer Nelson and a future first- and second-round draft pick.
The Mavericks' starting point guard trio of Jameer
Nelson (33 games), Devin Harris (three games) and J.J. Barea (1 game) have
ranked in the bottom of the league in several categories. On the other hand,
Rajon Rondo leads the league with 10.8 assists per game.