East Rutherford, N.J ? For Jim O’Brien, Paul Pierce and the rest of the Boston Celtics, the key to their Eastern Conference final series with the New Jersey Nets is simple.

Play a little defense and the Celtics think they can win. Show up with less than a strong defensive effort, and the Celts’ season will be over soon.

New Jersey's Jason Kidd (5) shoots over Boston's Kenny Anderson during Sunday's game at East Rutherford, N.J. The teams meets again tonight at New Jersey.

Tonight at the Meadowlands, the Celtics will have a second chance to beat the Nets before the series shifts to the FleetCenter this weekend. After watching tape of Sunday’s 104-97 loss in Game 1, the Celts insist they’ll be an improved team tonight.

“It will come down to if our halfcourt defense, which I think is better than it showed, is up to the task of stopping New Jersey. That will be the story of the series,” said O’Brien. “We will find out very shortly.”

Why weren’t the Celtics ready Sunday to slow the Nets’ high-powered offense?

“Damned if I know,” O’Brien said.

Pierce couldn’t offer concrete answers either, but, clearly, the Nets caught the Celtics off guard. With time to look, listen and learn, Pierce said the defense will be better. It has to be.

“I don’t think we can play any worse defensively,” he said. “That’s what this series will be about, which team holds the opponent to a lower shooting percentage.”

Antoine Walker agreed, but offered a look into the problems when he noted that the Nets came out running (9 of 11 first-quarter field goals were either dunks or layups) and had the Celtics’ halfcourt defense guessing.

“In the halfcourt sets, it took us a while to figure them out. They had a lot of backdoor cuts and a lot more movement than we’ve seen before,” he said. “They were cutting harder and they were very physical. It’s a credit to them that they came out aggressive like that.”

While the Celtics spent a short practice at Seton Hall’s Walsh Gym watching their mistakes on tape and then walking through one breakdown after another, the Nets took the day to bask in the glow of their big win. The New Jersey-New York area is catching on to the Nets, long an NBA lightweight. With guard Jason Kidd leading seven double-figure scorers in Game 1, the Nets’ focus is on grabbing what everyone expects to be an even tighter game tonight.

“If, we go up 2-0 with a little momentum, it will be tough for them,” said reserve guard Lucious Harris.

“If they get one and it’s 1-1, that really makes it harder on us.”

O’Brien put his team’s task in focus when he said: “Game 2 is very important and gives us an opportunity to do what we came to New Jersey for, which is to go back to Boston with the homecourt advantage.”

While O’Brien seemed more concerned than usual about his team’s defense, the players were writing the game off as one bad night.

They’re also assuming the Nets can’t play much better. That theory may have some legs, since the 104 points were the Nets’ second highest of the playoffs, where New Jersey has averaged 94.7 points per contest.

The players also note that they rallied in the fourth quarter to cut the Net lead to six points with 1:21 left.

Walker says the Nets “played their A-game.”

“I felt we did well. We battled and fought off everything they had. We took their best punch, man. They don’t get much better than they were. Seven guys in double figures, Jason Kidd a triple double. Todd MacCulloch (14 points) playing big. If we get seven guys in double figures, we don’t get better than that. We just have to make sure they don’t do that in Game 2.”