"When he [coach Doc Rivers] tells me I'm ready," Cassell said before the game. "If he says, 'Sam, I need you tonight,' I'm ready tonight."
The 38-year-old Cassell signed this week after being bought out by the struggling Los Angeles Clippers, but reports of an impending deal had been circulating for a month. He was expected to be with the team on Wednesday, but he left Boston to attend a family funeral.
At Friday's shootaround, his new teammates let him know they were waiting.
"Most of them said, 'About time,' " Cassell said. "This thing was dragging on."
Cassell has not played since spraining his right wrist on Feb. 20; he also missed 12 games earlier in the season with a strained left calf. When he gets on the court, he is expected to serve as a veteran presence and backup for point guard Rajon Rondo.
"He's a high IQ player," Rivers said. "I think he's going to be a great fit."
Rondo said he had no concern about losing his job to Cassell, joking that he would switch to shooting guard if necessary. Rondo, who's in his second year, is averaging about 11 points, five assists and four rebounds this season.
"I understand what he brings," Cassell said. "I'm not here to compete for his job; I'm here to be his backup. I want to make that clear first and foremost: He is the starting point guard for the Boston Celtics. ... I don't want his job; I'm here to make his job easier."
Cassell said he would push Rondo to be better and show how to prepare for the playoffs. Cassell won two NBA titles with the Houston Rockets, and the fact that the Celtics have the best record in the league was a big reason why he signed with them.
"This team has a great chance," he said. "Why not be a part of it?"
"I know what Kevin brings to a basketball team," Cassell said. "He made me an All-Star -- well, he helped make me an All-Star. Also, he became MVP that year I played with him in Minnesota."