When the annual showdown between Boston and Philadelphia for the NBA Eastern Division championship began 12 days ago, few would have anticipated Gerald Henderson being the catalyst in the Celtics' comeback hopes.

Henderson, a slender, third-year reserve guard from Virginia Commonwealth, played only 87 minutes in last season's seven-game series with the 76ers, finishing with 27 points and 13 assists. His role again this year was mainly to give Nate Archibald a breather once in a while.

Now Archibald is sidelined with a dislocated shoulder and the favored Celtics trail the 76ers, 3-2, in the best-of-seven series. Boston is relying heavily on Henderson when it tries to defeat the 76ers in Philadelphia tonight (WDVM-TV-9, 11:30, delayed) and force a seventh game Sunday in Boston.

When Archibald was injured last Saturday in the opening minute of Game 3, the Celtics were understandably jarred out of synch offensively and defensively. With their playmaker absent, they failed to score 100 points in either of two losses that weekend in the Spectrum.

With Henderson feeling more confident after two days of practice as the new floor leader, the Celtics' offense returned to peak efficiency in Wednesday night's 114-85 romp. Boston Coach Bill Fitch is hoping it remains at that level the rest of the series.

"Those two days of practice really helped," Henderson said, referring to lengthy workouts Monday and Tuesday. "We established a game plan that I felt comfortable with. I knew what the coach wanted and I didn't have to look over at the bench every time I got the ball.

"We were a lot more fluid," he continued. "We had five people moving as one, instead of five people going in five different directions like we were in Philly."

One of Henderson's major assignments was to get Robert Parish back in the flow of the offense. The 7-foot center, who averaged 23 points a game when the Celtics defeated Washington, 4-1, in the conference semifinals, had only 32 points in those weekend losses. He was benched for the fourth quarter of both games.

"I want to go to Robert early," Henderson said. "The best way to do that is push the ball up quickly before the defense can get set. If the defense isn't set, they can't help out on Robert and he's murder going one-on-one with anybody."

Parish scored 15 of his 26 points in the first quarter as the Celtics raced to a 33-22 lead they never lost. The game plan tonight is to follow the same formula. The pattern of this series, except for Game 3, has been that the team leading after the first quarter has won.

Another explanation for the Celtics' offensive resurgence is that Parish was positioned at the high post. This tactic will be repeated tonight for several reasons:

With Parish setting up near the foul line, forwards Cedric Maxwell and Larry Bird have more room to maneuver around the basket. Both had four offensive rebounds in Game 5 and scored 15 and 20 points, respectively.

Parish is more effective when facing the basket because he can shoot his jumper or drive, and doesn't have to trade elbows with Caldwell Jones or Darryl Dawkins.

By playing Parish outside, Jones or Dawkins are drawn from the basket and are less effective rebounders and shot blockers. Dawkins had one blocked shot Wednesday and Jones had none.

"Facing up helps me," Parish said. "They don't know if I'm going to drive or shoot the jumper. It puts a lot more pressure on the defense."

Pressure is something the 76ers have been fighting since they left their cramped dressing quarters in the Boston Garden Wednesday night. The question reporters were asking and the players must be asking themselves is: will this be a repeat of last year's series, when the 76ers took a 3-1 lead and then lost three in a row?

"I won't be thinking of it," Julius Erving said, unconvincingly. "What happened last year is history."

Only four teams have come back from a 3-1 deficit to win an NBA playoff series. The Celtics did in 1968 and last year, Los Angeles overcame Phoenix in l970 and the Bullets won three in a row over San Antonio while trying to repeat as champions in 1979.

If the 76ers fold again, a wholesale housecleaning by Harold Katz, the new owner, might ensue. Such a move could end Erving's hopes of winning an NBA championship. The 32-year-old forward has played six years in the NBA, having made it to the finals in 1977 and 1980.