Bass blamed poor positioning for the big disparity in offensive rebounds.
"We tried to help and got ourselves out of position," Bass said. "Their bigs were naked under the basket for the most part."
While it didn't work out the way he'd hoped, Stevens tried to stay positive.
"At the end of the day we didn't do everything perfectly," he said. "I didn't coach a perfect game, but I think we can all rest assured we've got a team that will fight and we've got a team that will compete. And we can shore up a couple of those mistakes, maybe we can come out the other end of it."
Bass echoed his coach's assessment.
"Being a new team, it's a game we can build on," he said. "There's some things we can definitely get better at and some things we can build on."
Humphries, however, found it harder to see the silver lining.
"It's the kind of (opposing) team and kind of game we've got to figure out a way to win," he said. "I know it's the first game of the year, but I'm a little disappointed. I was hoping we'd get this."
Stevens, who joined the Celtics after two Final Four appearances at Butler University, said he felt good about his first regular-season game as an NBA coach.
"No question it feels different, but it feels night and day better than it did the first exhibition game," the 37-year-old Stevens said. "I felt pretty good out there all night, at least felt like I knew what was coming and going."
That wasn't the only encouraging sign for the Celtics, who fell behind by 16 points in the third but rallied to tie it at 71-all heading into the fourth.
After Boston guard Jordan Crawford tied it at 78-all on a jump shot with 7:42 left, the Celtics didn't score again until Green's jumper with 4:08 remaining.
Johnson answered with a layup, Gay hit a turnaround jumper and DeRozan banked in a jump shot as Toronto opened a 10-point cushion, 90-80, with 2:34 left.
"We played pretty good throughout the game, but we played better in the fourth," Gay said. "We could have played better but a win is a win."
Faverani scored nine points in the first and the Celtics made 11 of 17 field goal attempts to lead 26-21 after one quarter.
Boston went cold to start the second, missing eight of their first nine attempts and allowing Toronto to take the lead with an 8-1 run.
The Celtics made just two of 14 shots in the second, while the Raptors got six points from Terrence Ross and four each from Gay and Johnson to lead 49-37 at the half.
Toronto extended its lead to 16 points at 60-44 on a Valanciunas dunk at 8:18 of the third, but the Celtics rallied behind nine points from Green and 10 from Bass, whose buzzer-beating dunk knotted the score at 71-all.
Boston forward Jared Sullinger served a one-game team suspension, two days after a judge dismissed domestic violence charges against him. ... Singer Jon Bon Jovi and former Raptors star Morris Peterson attended the game.
Copyright by STATS LLC and The Associated Press
TORONTO, CANADA - OCTOBER 30: Vitor Faverani #38 of the Boston Celtics drives to the basket against the Toronto...
MVP: Rudy Gay. Toronto's prized small forward scored seven of his 19 points in the fourth quarter, helping the Raptors pull away late against a pesky Celtics squad.
X factor: Offensive rebounds. Toronto took advantage of Boston's undersized frontcourt, as well as Jared Sullinger's suspension, to wreak havoc on the glass. Led by newcomer Tyler Hansborough, the Raptors racked up 19 offensive rebounds, leading to 28 second-chance points.
That was ... sloppy: Early-season rust was apparent for both teams, as they combined for 39 turnovers. Boston was unable to cut its miscues late (six in the fourth quarter), which allowed Toronto to escape with the win.