"I know we're a better team than those two teams, I know we are," said Paul Pierce, who's last-second attempt clanked off the rim as the buzzer sounded. "It's just mentally coming out and having the right mindset. I don't think we have the right mindset coming into these games against opponents that we're supposed to beat."
Boston has surrendered 106.3 points per game in its previous three road games after limiting teams to 88.3 over the first four, and Kevin Garnett knows that the team needs to bring a better defensive effort going forward.
"We can't just show up. We've got to play some 'D,'" Garnett said after the game. "These teams we're playing against are very high-caliber offense teams. We know what they are. On paper they might not be whatever but as far as talent, this league has a lot of talent and you've got to respect that."
The Celtics (9-4) closed out the first quarter against the Raptors with an eight-point lead, but gave up a season-worst 38 in the second, where they have been outscored in each of their four losses.
"We may be the worst second quarter team in all of basketball," said Pierce, who scored 19 in the loss. "We come out, get these leads and then we give them right away in the second quarter. We've got to do a better job and be more consistent throughout the game, quarter by quarter."
Pierce leads the team in scoring with 20.5 points per game and is averaging 23.7 in Atlanta over his career.
Rajon Rondo, who is averaging an NBA-best 14.3 assists, was held out of Sunday's loss with a hamstring injury, and his status versus Atlanta remains unknown.
Nate Robinson replaced Rondo in the starting lineup and scored a season-high 22 points.
The Hawks (8-5), meanwhile, have had trouble scoring at home, where they are averaging 98.3 points compared to 104.5 on the road.
They have lost four of five at Philips Arena, including a 98-93 loss to Dallas on Saturday night.
Atlanta trailed by as many as 18 before a late comeback fell short.
"We didn't come out with the energy we needed," said Joe Johnson, who matched a season low with 11 points for the second consecutive game.
Since 2008-09, Johnson is averaging 23.0 points against the Celtics, and is likely to come out with another strong performance against them.
Al Horford scored 20 points and pulled down 20 rebounds against the Mavericks for the second 20-20 game of his career. Horford, who is scoring a career-best 17.4 points per game this season, has been limited to 10.1 over his previous eight games against Boston.
The Hawks are shooting 29.8 percent from three-point range at home -- 25th in the league.