As Cassell entered, his teammates responded with cheers and applause that could be heard behind closed doors.
Cassell and Boston's backups outplayed Atlanta's starters in the fourth quarter, and the Celtics prevented the Hawks from clinching their first playoff berth since 1999 with a 99-89 victory Saturday night.
Atlanta would have ended the NBA's longest playoff drought with a victory, because Indiana lost to Charlotte earlier Saturday. The Hawks remained two games ahead of the Pacers with two to play, lowering their magic number to one for the right to face the Celtics in a first-round series.
Cassell, 38, scored 20 points, including 15 in the fourth quarter, as the Celtics outscored the Hawks 21-5 down the stretch.
"Sam's performance was vintage Sam," said Boston's Paul Pierce. "He's been playing really well as of late ... and he's going to be big for us in the postseason."
The Hawks scored only 15 points in the final period, including nine on three 3-pointers by Mike Bibby.
Cassell began the season with the Clippers before joining Boston as a free agent on March 4. He downplayed setting his scoring high since joining the Celtics.
"Just doing my job," said Cassell.
"It ain't easy at my age. But I'm getting a nice little rhythm right now. I'm understanding where I can fit in on this team offensively. Defensively, it's still a challenge for me."
Each team was playing for the second straight night, but Boston, which has secured the home-court advantage throughout the playoffs, had the luxury of relying on its bench.
"At halftime I told our coaches I thought we honestly had a better chance to win tonight with our second unit than our starters," Boston coach Doc Rivers said. "You could just see it, in that third quarter, that their energy was low.
"I said, 'Listen guys, I don't know if we're going to win or lose, but we're not going to play this way."'
The Hawks rallied from a 55-44 halftime deficit for a 74-74 tie after three quarters.
Atlanta also opened the final quarter strong, taking an 84-78 lead on two 3-pointers by Bibby before Cassell took over.
"I wouldn't say it was their second unit that beat us," Smith said. "It was Sam Cassell. He was pretty much the only player on the court who could score points."
Cassell shot 6-of-9, making two 3-pointers, while playing the full final quarter.
"It was fun, but it was also demanding," Cassell said.
Garnett, Allen and Pierce each played fewer than 25 minutes in Boston's 102-86 win over Milwaukee on Friday night, but the three passed that mark in the third quarter against Atlanta before the backups took over.
Joe Johnson scored 21 and Smith and Horford each had 17 for Atlanta. Bibby added 16.
Hawks coach Mike Woodson said he regretted playing Bibby only five minutes in the second period. Bibby hit two straight shots to give Atlanta a 35-34 lead but watched with two fouls the last 6:39 of the half as the Celtics used a 21-9 run to take a 55-44 lead at the break.
"If I had to do it all over again, I probably should have had Bibby's butt on the floor," Woodson said.
Bibby, who has not fouled out of a game this season, finished the game with only two fouls.
Rivers said the Hawks would not be an easy playoff opponent.
"Atlanta's tough. They're going to be tough," Rivers said. "I do believe that's who we're going to play in the first round. And it's going to be a tough series, it really is."
The Hawks, who play host to Orlando on Tuesday night before closing the regular season at Miami, took little consolation in playing even with Boston's starters.
"It would have been a lot better if we had clinched it tonight," Atlanta's Josh Childress said.
Added Smith: "Right now it's important for us to win. Tonight was a perfect opportunity."
- On the 50th anniversary of the St. Louis Hawks' NBA championship victory over Boston, Bob Pettit -- who scored 50 points in the decisive victory on April 12, 1958 -- was honored at a halftime ceremony. Pettit's No. 9 is one of only three numbers retired by the franchise.
- The game was a sellout, drawing 20,098.
- The Celtics swept the three regular-season games with the Hawks, including two games in Boston.