BOSTON – Boston coach Doc Rivers knew his team was exhausted after surviving a grueling first round, seven-game series with the Chicago Bulls, so instead of having his team practice on Sunday to prepare for the Orlando Magic, he gave the Celtics the day off.

Boston’s lack of preparation was evident early in Game 1 Monday night, and as a result, the Celtics found themselves down by 18 points at halftime and 28 early in the third quarter.

The Celtics gave a valiant effort over the final 16 minutes of the game by picking up their intensity on defense, forcing turnovers, making better decisions with the basketball and getting to the foul line. But they came up short.

Nonetheless, the message was sent. The Celtics had several opportunities to win even after playing poorly.

They say that won’t happen in Game 2, and Rivers refused to use Boston’s fatigue as an excuse.

“There was no fatigue,” Rivers said. “I don’t believe in that. We had a whole day off, we ain’t making no excuses. There was no fatigue. We played flat. We played with no energy in the first half, but it wasn’t the fatigue factor.”

The Celtics hope to carry the momentum they gained in the second half into Game 2, which is a game that the Celtics are considering a must-win. No team in franchise history has ever come back to win a series after falling behind 2-0, so the stakes are high.

“We just have to keep the momentum going that we gained in the second half,” Boston forward Glen Davis said. “At the beginning of the game, we have to start off strong and hit them in the mouth first.”

“We have to build on it. It’s a long series,” Boston guard Stephon Marbury said. “It’s seven games. For us, we have to continue to play with that mindset that we played in the second half.”

Boston captain Paul Pierce said: “It’s all about energy and effort because when we went in at halftime, we really didn’t change anything. We didn’t alter our game plan. We just did everything we were supposed to a little bit harder and that’s what got us back in the game.”

Rivers admitted before Game 1 that he gave his players Sunday off to get their legs back under them because he felt that was more important than going over Orlando’s plays. After all, the two teams met four times during the regular season so the two teams were familiar with one another, and Boston’s coaching staff gave its players an Orlando playbook to study.

But the Celtics looked lost at times on both ends of the court, especially in the second quarter. The Celtics watched a lot of film on Tuesday at practice, and they believe they will be much more prepared to get off to a better start in Game 2.

“We know that they are going to run,” Boston guard Ray Allen said. “We have some understanding of their plays and where they are trying to go. Obviously there is no surprise because we played them several times this year, so we just have to put that effort back out there.”

The Celtics were only down by five points after the first quarter. But the Magic closed out the first half with a 26-6 run to take control.

“They penetrated us. They scored in transition. They got to the paint. They were more aggressive on offense and defensively we kind of let them do what they wanted to. That sums it up,” Allen said.

“We have to play better and that’s the bottom line,” Rivers said. “We have to execute better on both ends. We have to play with more speed as a team. Speed is power and we didn’t play with a lot of speed until late in the game. The execution was bad. The defense was bad. The offense was bad.”

Orlando’s players said that they got complacent in the second half after they went up by 28 points. They settled for jump shots and played hesitant, allowing the Celtics to get back into the game. Like the Celtics, the Magic say that they need to play aggressively for the full 48 minutes, and if they do, they’ll be victorious in Game 2.

“We’re up 1-0 in the series,” Orlando forward Rashard Lewis said. “Getting a win on their home court is really big for us. We just gotta learn how to keep a lead and win, but obviously we got the win and we’re happy about that.”

“We had a great start but in the second half we just had so many turnovers and made bad decisions,” said Orlando forward Hedo Turkoglu. “That’s the thing we have to work on, try to control the whole night long instead of just [the first half].”

Orlando coach Stan Van Gundy said that the Magic will do everything they can to prevent the Celtics from carrying over the momentum they gained in the second half of Game 1 into Game 2.

“I’m sure the last 16 minutes gave them tremendous confidence that they can run all over us, but I think we can go back and take a look, see the mistakes we made, and change the way we played and play a better game throughout on Wednesday.”