By Larry Fine

NEW YORK (Reuters) - The Cleveland Cavaliers, who had the NBA's top record in the regular season, suddenly have their backs against the wall and need league MVP LeBron James to step up or else they will take an early playoff exit.

James behaved more like a passive bystander than the best player on the planet in the last three quarters of Tuesday's 120-88 home-court loss to the Boston Celtics in a game that gave Boston a 3-2 lead in the best-of-seven series.

National basketball team coach Mike Krzyzewski said he expects a bounce back from James, who helped the coach lead the squad to Olympic gold at the 2008 Games in Beijing.

"LeBron's a winner and when you lose you hate it. When you lose the way they did you really hate it," Krzyzewski said in an interview Wednesday with ESPN radio. "I would think (LeBron would) come out in an amazing fashion in their next game."

The lackluster performance by James, who scored 15 points on three-for-14 shooting, clouded expectations of an NBA Finals pitting the Cavs star against last year's MVP Kobe Bryant, winner of four championships with the Los Angeles Lakers.

Besides a first NBA crown for the man known as King James, his playing future in his home state of Ohio might also be hanging in the balance.

Boston, the 2008 NBA champions, came alive with a team effort sparked by their aged Big Three.

Ray Allen, 34, led the scoring on Tuesday with 25 points. Paul Pierce, 32, added 21 points and 11 rebounds, while 33-year-old Kevin Garnett contributed 18 points.

The Magic, who lost last year's NBA Finals 4-1 to the Lakers, have looked formidable trouncing the Atlanta Hawks by an average of 25 points a game after also sweeping the Charlotte Bobcats in the opening round.

Los Angeles and Phoenix will open their best-of-seven Western Conference finals on Monday.

(Editing by Frank Pingue)