LOS ANGELES — Kendrick Perkins went out, and the Boston Celtics' toughness went right along with him.

Perkins injured his right knee midway through the first quarter of Game 6 of the NBA finals on Tuesday night, and the Los Angeles Lakers capitalized by controlling the paint and the backboards after he left.

Los Angeles won 89-67 to force a Game 7 on Thursday night, and the Celtics will spend all the time they have until then hoping their starting center will be available.

Because without him, they were the weaker team, outrebounded 52-39 and getting outscored by more than a 2-to-1 difference on second-chance points.

Perkins lacks the importance of Boston's Big Three or fellow starter Rajon Rondo, who has been the Celtics' best player during an unlikely run to the NBA finals.

But he is crucial to the Celtics' defensive schemes because he's solid enough to play his man 1-on-1, as Boston had him do against Orlando All-Star Dwight Howard in the last round, and is a good help defender and rebounder.

That's become even more important this season because Kevin Garnett can't do the things he did two years ago, when he was the Defensive Player of the Year and a quarterback of Boston's defense.

Perkins was going for a rebound when he was fouled by Andrew Bynum with 5:30 remaining in the first quarter. He quickly fell to the floor and remained there for a few minutes before a pair of teammates slowly helped him to the back for X-rays, which revealed a sprain.

The Lakers led by six when Perkins went out and easily added to their advantage, fearlessly attacking the rim for powerful dunks from the likes of Shannon Brown and Jordan Farmar without a big body there to stop them — or make them pay for their audacity.

The Celtics have other big men, but starting Rasheed Wallace or Glen Davis weakens a bench that played such a key role in their Game 4 victory. Whatever situations they use Wallace in, he will have to be better than he was Tuesday, when he missed all seven shots and was 0 for 6 from 3-point range.

The fiery Perkins came into the finals with six technical fouls during the postseason, one away from an automatic suspension. So the Celtics have faced the fear all series of potentially being without their center at some point.

They might have to go without him Thursday in the game that will determine their season.

They will have to be better and tougher than they were Tuesday.