The 39-year-old Cassell hasn't played this season for the Celtics, who acquired him in early March before their run to the NBA title. He essentially served as another assistant coach this season with Boston, which also sent cash considerations to Sacramento in the trade.
The deal on Tuesday also decreases Boston's luxury tax obligations for the season.
The Kings, who have the NBA's worst record at 11-43, didn't immediately announce whether Cassell would join the team or be released. Sacramento hosts the Atlanta Hawks on Wednesday night.
Cassell has won three NBA championships during his 16 seasons in the league.