BOSTON — The Celtics have been saying all season long that they are a team.

They’ve been saying to everyone who will listen that they believe in each other and that they can’t rely on just one, two, or three players to get them to their goal — the NBA championship.

For all the hype about Boston’s Big Three, it was their bench that got them back into the game last night after Boston’s starters fell behind by 12 points in the first quarter. Their surge helped rally the Celtics past Cleveland, 89-73.

“We gave our team some momentum, and that’s what it’s all about,” Boston forward P.J. Brown said of Boston’s second unit. “There are going to be times when the starting group is going to [struggle] — it’s going to happen, and it’s our job to come off the bench and be ready to go out there and get us a win.”

Boston backup center Leon Powe scored 11 points with 7 rebounds, backup point guard Sam Cassell netted nine points, backup forward James Posey notched seven points with six rebounds, and Brown recorded four points and two rebounds in less than six minutes of action.

“We just have to be ready when our name is called,” Powe said of Boston’s second unit. “We just go out there and play 110 percent — all out. It’s the playoffs. We don’t have time to rest so when you are out there, just go all out.”

Boston’s bench players outscored its starters, 26-18, in the first half. Only two Boston starters scored in the first half –– Kevin Garnett and Paul Pierce with nine points apiece. Boston led at halftime, 44-36.

“Our job is to go in there and turn the game around or try to make something happen,” Powe said.

Trailing 21-9 with 2:49 remaining in the first quarter, Boston coach Doc Rivers inserted Cassell and Posey in for Rajon Rondo, who was 0-for-3 at the time with two turnovers and only one assist with no points, and Ray Allen, who was 0-for-1 from the floor with one steal, one turnover, and no points.

Powe, Cassell and Posey helped power the Celtics’ comeback by providing the spark that Rivers was hoping for. They helped the Celtics cut Cleveland’s lead to 24-17 by the end of the quarter. Cassell and Powe combined for six of the final eight points in the quarter.

“Guys like [Cassell] and Posey have been to the mountain top,” Brown said. “They are two guys who know what it means to win and who know that feeling to win it all. We are fortunate enough to have two veteran guys like that on our team.”

Powe had a monster first half, scoring nine points with six rebounds. Both numbers were more than he averaged in the regular season (7.9 points, 4.1 rebounds) and playoffs (6.0 points, 3.5 rebounds) for an entire game.

“The sky is the limit for Leon,” Brown said. “He’s a hard worker. He brings his best in practice and he’s a focused young man who comes out and gives his best every night.”

Cassell, Powe, Brown and Posey combined to score Boston’s first 15 points of the second quarter, when the Celtics outscored the Cavaliers by 15-6, which put Boston ahead 32-30. Boston never trailed after that.

“We know what we have to do,” Powe said of the second unit. “If we have the lead when we come in, we have to keep it or increase it. If we don’t have the lead, we have to do something to get the lead. We know what we have to do and by us doing that, it helped us win the game.”

roblee@projo.com