CLEVELAND – His team down by 27 points and a promising season suddenly in peril, LeBron James walked off the floor to a smattering of boos and rows of empty seats Tuesday night.

If this was goodbye Cleveland, it wasn’t the send-off he imagined.

The Boston Celtics have pushed James, the two-time MVP, and the Cavaliers dangerously close to an early start to the Summer of LeBron.

Ray Allen scored 25 points, Rajon Rondo scored all of his 16 in the second half and the Celtics, once thought too old to challenge for another title, beat James and the Cavs 120-88 in Game 5 to move within one win of knocking the league’s top team from the playoffs.

Paul Pierce added 21 points and Kevin Garnett 18 for the Celtics, who handed the Cavs their worst home playoff loss in history and can end Cleveland’s season Thursday night with a victory in Game 6.

“We cannot come back here,” Garnett said. “We have to think this is our Game 7 coming up and we cannot afford to have the best team in the league have a Game 7 on their floor. Just not possible.”

James, on the verge of an expected trip into free agency July 1, had an atrocious game. He scored 15 points on 3-of-14 shooting. He refused to use his sprained elbow as an excuse.

“I missed a lot of open shots that I normally make,” he said. “You don’t see that out of me a lot so when it happens, it’s a big surprise.”

Because of James’ uncertain future, Game 5 may have been his last at home for Cleveland and it has set up Game 6 as the most important in franchise history: Win and force Game 7 on Sunday in Cleveland; lose and maybe watch James, the local kid trying to deliver this city its first pro championship since 1964, leave for good.

“Our backs are against the wall,” James said. “We’ve won on that floor before and we’ve got to get it done.”

As for this potentially being his last game in Cleveland, James added: “I didn’t even think about that. Me sitting up here and saying this is potentially our last game here this season, that wouldn’t be me and that wouldn’t be our team.”

Rondo, coming off a 29-point, 18-rebound, 13-assist performance in Game 4, was held without a point in the first half as the Cavs concentrated their defense on stopping him from penetrating into the paint. He finally got loose in the third, scoring 12 points as the Celtics opened a 21-point lead.

Boston went up by 24 in the fourth. James finally checked out with 3:58 left and the Celtics leading by 27. He shrugged his shoulders and slapped hands with Cleveland’s coaches and teammate Shaquille O’Neal, who had 21 points and afterward stated the obvious.

“It’s plain and simple,” said O’Neal. “We’ve got to win two in a row. We’ve got to man up. I’ve been in this situation before.”

Before the game, Celtics Coach Doc Rivers said his team wouldn’t change its strategy.

“We are who we are,” Rivers said. “We don’t need anyone to play hero basketball. We have to be a team. We’re good when we’re a team.”

And through five games, the Celtics have been the better one. Because of injuries, Boston, two years removed from its 17th NBA championship, never found its groove in the regular season.

The Celtics have it now.

They’ve outperformed the top-seeded Cavs in almost every aspect of the game, outrunning and outhustling a younger team that with the addition of O’Neal, Antawn Jamison and Anthony Parker, was built for the postseason but has yet to show it’s serious about winning a title.

“We’ve done nothing,” Rivers said. “We’ve won three games but we’ve got to win four.”

Allen opened the second half with back-to-back 3s, pushing Boston’s six-point halftime lead to 12.

In the first half, the Cavs did a brilliant job on Rondo, who didn’t score his first points until 9:47 of the third quarter. But by then the Celtics had opened their double-digit lead.