BOSTON: It is a simple basketball premise that even at the highest level is acutely true: You cannot win if you cannot score.

Both the Cavaliers and the Boston Celtics believe in the matching inverse, which is to win by not letting the other guy score. Right now, the Celtics are doing it better and they are halfway to the next round of the playoffs because of it.

Behind a masterful defensive performance and complementary offensive contributions from numerous players, the Celtics defeated the Cavs 89-73 in Game 2 of their Eastern Conference Semifinals series Thursday.

The Celtics now have a 2-0 series lead. As if their No. 1 seed and star power didn't already make it so, history says the Celtics are big-time favorites to advance. Only 19 teams in NBA history have come back from losing the first two games to win a series. The Cavs did it last year against the Detroit
Pistons, but the way things are going, that is merely a footnote.

Using excellent principles, overall athleticism and surging momentum, the Celtics have successfully flustered the Cavs when they have the ball. Already a fragile offensive team that doesn't handle pressure well, the Cavs' poise has been shattered by the challenge of dealing with the league's best defense.

Look no further than LeBron James, whose promise of not having two bad games in a row did not come to fruition. After going 2-of-18 in Game 1, James was only marginally better in Game 2. This time, he made 6-of-24 shots on his way to an ineffective 21 points.

''They have athletic big men and they are doing a good job of rotation,'' James said. ''They don't allow me to crack the second line of defense.''

The last time James was reduced to this in a series was against the San Antonio Spurs in last year's NBA Finals, when he admitted he ran into a better-prepared and better- equipped team. He is not ready to make another such proclamation, but that could be mere days away. The Cavs shot 36 percent Thursday and are now at 34 percent for the two games.

It is a combination of factors. The Celtics are a long and athletic team, starting with Kevin Garnett and the two men who primarily deal with James, Paul Pierce and James Posey. They help each other well and trust each other, which are two rock-solid defensive principles.

This makes them a tremendous pick-and-roll defensive team. Yet knowing this, the Cavs ran pick-and-roll after pick-and-roll as James was left dribbling into crowds, putting up low-percentage jumpers or searching for teammates who were not open.

''This is what we work on,'' Pierce said. ''We go to practice and we work on trying to contain him. LeBron is what makes them go. If we control him, then we can control them.''

In the first quarter, the Cavs made 10 of their first 16 shots and took a 12-point lead. Six of those buckets came from Zydrunas Ilgauskas, and most of those came from driving and passing, in-and-out action that made the Celtics move and not merely set up. The Cavs had 10 assists in the quarter, and their between-game adjustments looked to be working.

But the Cavs' offense has been temperamental all season, the Celtics are unrelenting, and as the ball stopped moving, so did the Cavs' score. They managed 12 points on 2-of-17 shooting in the second quarter. They went through a stretch where they went 3-of-24 as the Celtics turned it into one of their trademark home blowouts in the second half.

Ilgauskas became an afterthought. It took 20 more minutes of floor time for him to get his next six shots. He ended up making 9-of-12 for 19 points, the only Cavs player to get it going at all on offense.

Not true for the Celtics, who seemed to have recaptured the rhythm that earned them 66 wins in the regular season, Pierce was much more efficient than James, scoring 19 points on 13 shots. Garnett was solid with 13 points and 12 rebounds. Ray Allen didn't score in the first half, then poured it on with 16 in the second half.

Leon Powe had 11 points off the bench as the Celtics' reserves outscored the Cavs' 34-17.

The Cavs didn't play bad defense; they just couldn't survive all the misses. Plus their 15 turnovers were turned into 23 points by the Celtics.

The worse news for the Cavs was Ben Wallace had to leave the game in the first quarter after a bout of dizziness, which might have been caused by the smoke around the court following the pregame fireworks. His status for Saturday's Game 3 in Cleveland is uncertain.

''Boston should be confident,'' Cavs coach Mike Brown said. ''But it's not over until somebody wins four games. Game 3 is its own entity. We've got to go out and get energized from our crowd and get a win.''