BOSTON — The Celtics passed their first grueling test of the year with flying colors.

The defending world champions went 7-1 in their tough stretch of playing eight games in 12 days.

“If someone would have told me before [the season] that we’d go 7-1 during this stretch, I’d say I’d take it,” Boston coach Doc Rivers said after Boston beat Milwaukee in overtime Saturday night.

The Celtics are 9-2 overall and are sitting in the same position that they were last season after 11 games — in first place in the Eastern Conference’s Atlantic Division.

But when comparing this year’s first 11 games versus last year’s, Boston coach Doc Rivers said there was one glaring difference.

Last year at this time, Boston was beating teams by an average of 14.1 points per game. They were averaging 103.3 points per game through their first 11 games, and holding teams to only 89.2 points per outing.

The defense is still there.

The Celtics have limited their opponents to just 89.7 points per game, and only one team (Atlanta) has reached the century mark against Boston this season.

“Everything starts with our ‘D,’ ” Rivers said.

But the Celtics have struggled offensively, mostly in the first half, where they have gotten off to slow starts. Opposing teams have outscored the Celtics seven times in the first quarter in their 11 games, and the Celtics lost one of the four games in which they outscored their opponents in the first quarter.

Boston is averaging only 94.3 points per game.

The problem, Rivers said, is that the Celtics are not playing for a full 48 minutes, which has resulted in the slow starts, the close games, and their two losses. The exhausting early-season schedule, one like Rivers said he’d never seen before, has been taxing on the Celtics.

“It’s going to catch you at some point — it just has to,” Rivers said about Boston’s slow starts and not playing the full 48 minutes. “I don’t care how good you think you are. If you’re going to play with fire, eventually you’re going to get burned.… We have things to fix.”

“We have to be prepared a little bit better with teams coming at us from the jump,” Boston captain Paul Pierce said. “…Teams are coming at us for 48 minutes, and we have to find a way to put 48 minutes together.”

The Celtics have talked extensively about playing a complete game, but no matter who you ask in their locker room, they’ll say it hasn’t happened yet. They also never used their schedule as an excuse for struggling at times in games this season.

“Well, we gotta just basically have a better focus before a game,” Boston guard Tony Allen said. “We know that coming into the game we are going to get everybody’s best and we just have to have a better preparation.”

“We just have to find ways to play better as a whole unit,” Boston guard Eddie House said.

Falling behind early in games has meant that the Celtics have had to play their Big Three for more minutes than they would like to, Rivers said.

Pierce, who is 31, is averaging 39.4 minutes per game. Allen, who is 33, is averaging 35.8 minutes per game (37.2 over his last six), and Garnett, who is 32, is averaging 33.5 minutes per game.

“We can’t go with Paul and Ray playing 40 minutes every night,” Rivers said. “Those numbers have to come down to the 34 minute mark.”

Garnett’s minutes have been down because big men Leon Powe (7.1 points, 3.4 rebounds), Kendrick Perkins (5.5 points, 7.3 rebounds), and Glen Davis (1.8 points, good defender) have been playing well.

“The bench is going to be big for us because [this] is when I start using the bench more,” Rivers said. “…I think our bench is capable.”

With their grueling stretch over, the Celtics got a well-deserved day off yesterday to rest. They go back to practice today at the Sports Authority Training Center in Waltham.

Rivers hopes that the Celtics will be reenergized and refocused to play a full 48 minutes tomorrow against the New York Knicks at the TD Banknorth Garden at 7:30 p.m.

“We’re not happy with the way we are playing, but that’s a good thing,” Rivers said. “We’re winning games, but we’re not playing well enough to be a winner, and we get that. I think our guys understand that.

“Last year, we were exactly in the same spot, so record-wise, it’s great, but we know that we are going to have to be better.”

roblee@projo.com