BOSTON — The Celtics were supposed to be too old, too hurt, too inconsistent.

Boston coach Doc Rivers knew better.

The team that muddled through its last 54 regular-season games with a 27-27 record is headed to the NBA finals after winning Game 6 of the Eastern Conference finals over the Orlando Magic 96-84 Friday night.

“The first thing we said when we got in the locker room is this is where we thought we would be,” Rivers said. “So don’t be surprised. This is what we talked about before the season started.”

An 86-77 win at Orlando on Christmas Day gave the Celtics a 23-5 record. Then injuries to Kevin Garnett and Paul Pierce took their toll and the Celtics were content to give them and others time to heal even if it meant risking losing games late in the season.

The strategy paid off and now Boston will play Los Angeles or Phoenix with a chance for its second championship in three years and 18th overall, an NBA record.

“That stretch the last month, we formed a game plan, and I thought it was the right plan,” Rivers said. “Obviously, it didn’t look right because we were losing games, but guys were resting and conditioning, and I thought that was the only chance we had.”

That mediocrity didn’t fool Magic coach Stan Van Gundy, not after the Celtics eliminated the Cleveland Cavaliers and LeBron James in six games. And certainly not after they won the first three games of the best-of-seven series against the Magic.

“They beat two very good teams, and made us both look like we weren’t very good teams,” Van Gundy said.

Boston’s “Big Three” of Pierce, 32, Garnett, 34, and Ray Allen, 34, all made key contributions Friday.

Pierce led the Celtics with 31 points and 13 rebounds, Allen scored 20 and Garnett added 10 points and 5 rebounds. Throw in Nate Robinson’s 13 points off the bench in the second quarter, and the Magic’s chances of becoming the first NBA team to overcome a 3-0 deficit — there were 93 others — were slim.

“I never really think negative about losing, or being up 3-0 and something tragic happening,” said Pierce, the MVP of the 2008 finals. “Even though one day it’s possibly going to happen. … I’m just glad we were able to prolong it one more year.”