Cassell has missed four straight games because of a sprained right wrist.
A point guard, he averaged 12.8 points, 4.7 assists, 2.8 rebounds and 25.7 minutes in 38 games. Cassell, 38, missed 12 games earlier in the season with a strained left calf.
In all, Cassell had missed 41 games over the last two seasons because of injuries and illnesses.
He is earning $6.1 million this season -- the final year of his contract.
"After weighing all of the options and after conversations with Sam we believe this is the best solution for this team," general manager Elgin Baylor said in a statement. "We know and appreciate the many contributions Sam made during the last three years, and we wish him nothing but the best."
Cassell has to clear waivers by midnight Eastern on Friday to be eligible to play for another team in the postseason.
Three weeks ago, Cassell denied a Boston Globe report that he was trying to buy out his contract so he could play for the Eastern Conference-leading Celtics.
But he made no secret that he was unhappy about the Clippers' mounting losses. Cassell has said he'd like to play another season before retiring and going into coaching.
If Cassell joins Boston, he would be reunited with Kevin Garnett, his former teammate in Minnesota.
Cassell's most productive season with the Clippers came in 2005-06, when he averaged 17.2 points and 6.3 assists. He helped them beat Denver in the first round of the playoffs before getting within a game of reaching the Western Conference finals.
He was a member of NBA championship teams in Houston in 1994-95.