Posey said accountability up and down the roster is the reason for Boston's success on defense.
"We've got guys that take responsibility when they mess up," Posey said. "We correct our mistakes, get back out there and make sure we're on the same page. We just cause havoc for the most part."
The Celtics won their second straight game without guard Ray Allen (sore right ankle). Allen, who remained in Boston for treatment, is expected to return when Boston hosts Detroit on Wednesday.
Rondo has stepped up with Allen out, collecting 31 points and 12 assists in his past two games.
"He's been tremendous," Pierce said. "It seems like he's getting better each and every game."
Celtics coach Doc Rivers said the key was shutting down Toronto's outside shooters.
"Right after the season last year we said the one thing we have to do better as a team is guard the 3-line, and we've done it all year," Rivers said. "When you do it against [Toronto], it's bigger because they are the best 3-point shooting team in the league."
Chris Bosh had 17 points and 13 rebounds as Toronto lost for the first time in five games.
"Their defense is very aggressive," Bosh said. "They are very long and athletic and the plays we usually make just weren't there. What we need to take from this game is to see how they play defense."
Jamario Moon had 13 and Kris Humphries had 12 for Toronto, whose 77 points were a season-low. Toronto's previous low was 84, set in a Dec. 7 loss at Boston.
"They played hard and took us out of what we wanted to do," Humphries said. "They made it tough for Jose [Calderon] and our guards to get the rhythm they usually get."
Toronto trailed 71-54 after three quarters before opening the fourth with a 7-0 run, cutting the gap to 10. But with Garnett back on the floor, he and Pierce scored six points in an 8-2 run that put Boston up 80-63 with 5:52 remaining.
"They made an adjustment and got a couple of buckets and started to make their way back," Garnett said. "We adjusted to what they were doing and never looked back from there."
The Celtics led 27-17 after the first quarter thanks to 10 points from Pierce and poor shooting by the Raptors, who were 4-for-17.
Rivers said he stressed the importance of a strong start at the team's pregame breakfast.
"I said, 'You've got to come out of the box with great intensity,' and they did," Rivers said. "You could see it on the floor and I thought that was the difference. We got a cushion and we were able to hold them off."
After Anthony Parker's 3-pointer at 2:50 of the first, the Raptors didn't make another field goal until a dunk by Humphries at 8:45 of the second, with just a pair of free throws by Bosh in between.
Humphries' basket ended an 11-0 run, but by then the Celtics led 35-19, widening the gap to 21 on a layup by Pierce and a 3-pointer by Tony Allen.
Boston led 50-36 at the half.
The start of the second half was delayed by 14 minutes because of a leaky vent in the ceiling at the Air Canada Centre, leaving water dripping onto the floor near the Celtics bench. ... Boston has won all three regular-season meetings against Toronto this season, and won a preseason matchup in Rome. ... The Celtics are 8-2 on the road.