UConn coach Jim Calhoun was in attendance and received an ovation when he was shown on the large video board above the court.
"I bet you he felt out place," Allen said. "Just sitting there, he could relax."
Allen had eight points, six rebounds and four assists. He made his only field goal in the opening seconds of the third quarter and set up Garnett for a jumper that gave Boston its first double-digit lead, 72-62.
"The whole time I was shooting, I was just thinking about seeing the student section," said Allen, who went 26-0 in Hartford while playing for UConn. "This is a building in my mind that was always blue and white."
It was the first time the Celtics played in Hartford since a preseason game in 1995. Prior to then, the Celtics played three regular season games a year in the arena formerly known as the Civic Center.
Celtics point guard Rajon Rondo had six points, four steals and eight assists. He set up Garnett twice with alley-oop passes midway in the third quarter, including a dunk.
"That was nice," Celtics coach Doc Rivers said. "Unexpected, actually, because Kevin got kneed in the calf in the first half so I didn't think he was running well. And all of a sudden out of nowhere, he becomes the old Kevin."
After missing a reverse layup early in the third quarter, Bosh vehemently protested to referee Tre Maddox after no foul was called. Later in the quarter, Bosh had some words for referee Jaimie Morales after picking up his fourth and fifth fouls in a 10-second span.
"It was for a moving screen," Bosh said. "I told them I have to actually move into somebody to get a moving screen. You know, it is what it is. ... It's preseason, it's practice for everybody."
In the second quarter, Raptors center Andrea Bargnani was assessed a technical foul for protesting a call.
"I'm not getting into the referee's performance," Rivers said. "They're doing the best that they can do in a really, really difficult circumstance. It's not like we're going to change as coaches. We're nuts. We're going to complain and do all that stuff, and we don't care who it is out there. We're still going to do that."