The Celtics’ James Posey, left, and Kendrick Perkins, right, kept Cleveland center Zydrunas Ilgauskas and the Cavaliers’ offense at bay on Tuesday night.

The Providence Journal / Glenn Osmundson

WALTHAM, Mass. — As the old saying goes: “Offense wins games. Defense wins championships.”

Coaches have been saying that for years, and Celtics coach Doc Rivers is a big believer in that philosophy.

The Celtics pride themselves on playing tough defense. It carried them through the regular season where they won an NBA best 66 games and held opponents to the second fewest points allowed in the NBA (90.3) and the lowest field-goal percentage (41.9).

Boston is allowing only 76 points per game at the TD Banknorth Garden this postseason, which is the lowest scoring average by a team in the first five home games of a single postseason during the shot clock era (which began in the 1954-55 season).

The Celtics swarming defense was in full display in Game One of the best-of-seven semifinal series against Cleveland. They hope for a repeat performance in Game Two tonight.

They might need it. Boston scored only 76 points Tuesday night.

“Offensively, we were a terrible-looking group,” Boston guard Paul Pierce said yesterday after practice. “We turned the ball over 23 times, shot 40 percent from the field. We did just about everything wrong you could do offensively, and what happens, the defense comes and saves the day.”

“As a team, we didn’t play well offensively, but we did what we had to defensively and that’s all that matters,” Boston guard Ray Allen said.

Boston’s stellar defense limited Cleveland to just 72 points and a lowly 30.7-percent shooting from the field. In addition, the Celtics forced 17 turnovers.

The most impressive thing about Boston’s suffocating defense was its ability to shut down Cleveland superstar LeBron James, who led the NBA in scoring with 30 points, 7.9 rebounds, and 7.2 assists per game during the regular season before averaging 29.8 points, 9.5 rebounds, and 7.7 assists in Cleveland’s first-round series against Washington.

James struggled mightily, scoring just 12 points on a dismal 2-for-18 shooting, with nine rebounds, nine assists, and a whopping 10 turnovers. His 11.1 percent shooting from the floor was a career low for him.

“We were just trying to keep a body on him at all times and try not to give him any air space,” Boston forward Leon Powe said. “We didn’t want him getting to the rack and we did a real good job of that. We kept him outside on the perimeter and made him a jump shooter.”

Of James’ 18 shots, only three were at the rim.

“That was the big key,” Allen said. “You can sit and worry about what you are doing offensively but defensively the game is still taking place and he was attacking and we made sure that we kept bodies on him and we knew he is a pretty good passer so we were just trying to switch it up and make sure that he didn’t get too comfortable.”

The Celtics consistently helped Pierce and James Posey when they were guarding James to keep him out of the paint and force him to shoot from the outside. James also missed some shots that he usually makes.

“I wasn’t satisfied with the way I played but to say that is my worst game, nah,” James said yesterday at practice, adding that he will try to go to the rim more before vowing to play better tonight.

Boston isn’t expecting James to play as poorly as he did in Game One, but the Celtics will do their best to contain him once again.

“He’s always a threat while he’s out there,” Posey said. “When he’s not scoring, he’s capable of making nice passes, finding his teammates. He’s just always a threat while he’s out there.”

Cleveland coach Mike Brown said yesterday that the Cavaliers have to do a better job attacking the rim to beat the Celtics.

“We have to make sure we’re attacking and having multiple drives in the same possession,” he said. “There’s some things we can do better to hopefully increase that (shooting) percentage.”

Boston coach Doc Rivers said that the Celtics rushed a lot of their shots on Tuesday and didn’t run the offense as well as he would have liked them to. The Celtics will have to be more patient on the offensive end tonight to be successful, Rivers said.

“We know we can take better care of the basketball,” Rivers said, referring to Boston’s 23 turnovers. “We had a lot of unforced turnovers [Tuesday] night. We never established the tempo.”

roblee@projo.com