Boston ? Andrew Bynum is altogether clear on this: He’ll play in Game 5 of the NBA finals tonight.

Beyond that, the key to the Los Angeles Lakers’ interior defense — and perhaps the team’s championship chances — can’t make any promises.

“I’m 100 percent sure that I’m playing come (today),” the Lakers’ 7-foot center said Saturday.

Bynum had fluid drained from his right knee after Boston evened the series 2-2 with a 96-89 win Thursday night. He was encouraged that the swelling had not returned since then. He had fluid drained from the same knee just before the series and the swelling reappeared.

He knows that will happen again.

“It’s going to come back after Game 5, for sure,” Bynum said of the game that starts at 7 tonight in Boston. “It’s guaranteed. But as long as I’m able to be out there and be effective, that’s what counts. And I know this game is the biggest game because it can change the whole series.”

Bynum played 28 minutes in the opener, 39 in the second game and 29 in the third. He had 40 points and 22 rebounds as the Lakers opened a 2-1 series lead. But he played just 12 minutes in Game 4 and sat out the last 14 minutes. His totals: two points, three rebounds, no blocks,

Without him, the Lakers let a 62-60 lead after three quarters slip away as Boston power forward Glen Davis took advantage of Bynum’s absence to score nine points in the fourth quarter. Two of Davis’ baskets helped turn a 64-all tie into a 70-64 lead.

“It’s rare that you see an NBA game won by one six-point run and that’s kind of what happened,” Bynum said. “The whole game was real slow, real nasty. We had the lead and we kind of, so to speak, choked.”

Had he played, it would have been much tougher for Davis to grab rebounds and make layups. Bynum’s health could have been the difference.