Cassell is headed to Baltimore to attend the funeral of a family member and won't be available when the Celtics host the Detroit Pistons on Wednesday night. He is expected to be available on Friday when Chicago visits Boston, but whether he plays will be the coach's decision.
"I'm in no hurry," coach Doc Rivers said after practice Tuesday. "We'll have some time to work on things in the next few days."
Cassell, 38, averaged 12.8 points for the Los Angeles Clippers this season before negotiating his long-rumored exit. He is expected to back up starting point guard Rajon Rondo and play some off-guard.
"Since I got traded here last summer, Sam was excited about this team's potential," said guard Ray Allen, noting the deal will take pressure off Rondo.
"Sam will give Rondo another gear," Allen said.
The 6-foot-3 Cassell is expected to take minutes from Eddie House, who has been backing up Rondo all season.
Rivers said he is still unsure when recently signed forward P.J. Brown will make his debut.
"Sam may play before P.J.," Rivers said.
Heading into Tuesday's games, the Pistons trailed the Celtics by 3½ games in the race for the NBA's best record.
Although Cassell couldn't wait to get out of Los Angeles, which is last in the Pacific Division, he will be missed.
"His loss takes away leadership to our team. He has been a class act to this team," Clippers forward Corey Maggette said. "He is an all-around good guy. He knows the game."
Maggette said Cassell's former teammates understood his desire to go to a contender.
"This is Sam's 15th season in the NBA and it is about business," Maggette said. "He put himself in position where he can go to a team that can win a championship. Boston is one of the elite teams in the league."
In a related move, the Celtics assigned rookie guard Gabe Pruitt to the Utah Flash in the NBA's D-League.