The Celtics’ Rajon Rondo seaches for an open teammate while guarded by Detroit’s Chauncey Billups in the first half.

The Providence Journal / Glenn Osmundson

BOSTON — When you ask Boston coach Doc Rivers who the most important player in the Celtics’ offense is, you might be surprised by his answer.

Boston’s Big Three —All-Stars Kevin Garnett, Paul Pierce and Ray Allen — come to mind first to most people, but Rivers says that second-year point guard Rajon Rondo is just as important as them.

“Well, we pretty much have given (Rondo) the keys all year, you know what I mean?” Rivers said earlier in the playoffs.

Garnett, Pierce, and Allen have all had big games in postseason losses, but when Rondo plays well, the Celtics usually win.

This series has been no different.

Rondo has had a roller-coaster postseason, normally playing well at home but subpar on the road. He had big outings in Games One and Three — both Celtics victories — and he struggled in Games Two and Four, both Celtics losses.

He did have 10 points, 9 rebounds and 8 assists in Game Two, but he was 2-for-9, and after he missed his first two shots, he passed up several wide-open jumpers, which hurt Boston’s offense.

Overall, Rondo’s numbers have been down in this series. He’s averaging 10.7 points, 6.4 assists and 4.1 rebounds in the playoffs. But prior to last night’s game he was averaging only 9.3 points, 5.8 assists and 4.0 rebounds per game in this series.

Rondo said he didn’t have Eastern Conference finals jitters.

“I don’t feel any pressure at all because my teammates have my back,” Rondo said. “But then again, it comes down to point guards who run the team. I’ll just go out there, try to run my team, and let Paul, Ray and KG make the plays.”

Rondo was considered the underdog in his matchup against Detroit All-Star Chauncey Billups. But Billups entered the series having missed two weeks because of a strained right hamstring and he hasn’t played up to his potential because of the injury.

Billups said he was determined to play.

“Well, I think at this juncture of the season, no, you can’t sit out,” Billups said. “I’m a leader of this team, one of the main leaders of this team, and no matter what’s going on, I feel like I’m better out there on the court. We’ve got a young guy, (Rodney) Stuckey, who’s playing great right now and … having a great series and great playoffs, period. So we’ve got to find ways to keep him aggressive, keep him playing well. But at the same time, I’ve got to play — I know that I’ve got to play good for this team to win. I understand that.”

Stuckey has been a spark plug off the bench for the Pistons. While Billups is averaging 11 points and 5 assists per game against the Celtics, which is well below his regular-season average of 17 points and 6.8 assists per game, Stuckey has boosted his regular-season average of 7.6 points per game to 10.3 points per game against the Celtics.

“I’ve just got to keep doing what I’m trying to do, just go out there and just be aggressive and just play hard and do what I do,” Stuckey said. “Chauncey will be fine. We know he’s probably not 100 percent right now, but he’s out there trying to do his best, trying to lead us and stuff like that. But I’m just going to continue what I’m doing, just going to the basket, being aggressive and trying to get my teammates open.”

Detroit coach Flip Saunders said he’d to put either Billups or Stuckey on the floor against the Celtics depending on who is running the offense better.

“It’s not just a matter of how (Billups) is playing; it’s how the team is playing,” Saunders said. “If he’s doing what he needs to do to initiate our offense, key our defense, contain people as far as defensively, then he’ll play. If he’s struggling with that, I will not hesitate at all to go with Rodney.”

Rondo said he was challenging himself to outplay both Billups and Stuckey.

roblee@projo.com