he NBA Eastern Conference final took an abrupt turn in favor of underdog Philadelphia today when Boston's floor leader Nate Archibald dislocated his left shoulder. The 76ers went on to edge the Celtics, 99-97, to take a 2-1 lead in the best-of-seven series, which continues here Sunday (WDVM-TV-9, 1 p.m.).

Archibald collided with Maurice Cheeks while diving for a loose ball after 1:23 had elapsed in the game. The 31-year-old playmaker will be sidelined for the rest of the playoffs.

"You dummy, you don't dive that way unless there's water in the pool," Coach Bill Fitch said he told Archibald when he realized that he had lost one of his most valuable players.

Gerald Henderson now takes over as the Celtics' playmaker, with Danny Ainge as his backup. Ainge, who did not play in Philadelphia's 121-113 victory Wednesday in Boston, today played 17 minutes and scored six points.

"Danny had his ups and downs today," Fitch said. "Considering the position he was put in, I thought he did a good job. He's a rookie, but he's had 60 games and I think he'll respond in this series."

Although the 76ers led the entire second half, the Celtics reduced a 10-point deficit to two and had three chances to send the game into overtime.

After Julius Erving (19 points) missed a short shot with 20 seconds left, the Celtics got Larry Bird an open 20-footer. His shot bounced hard off the backboard to teammate Cedric Maxwell. Maxwell had an open shot, but hit the rim. He grabbed his own miss and went up strong, but Erving came from the weak side and blocked the shot. The ball bounced back to Maxwell again, but Cheeks grabbed it away as the final buzzer sounded.

"I was afraid to look in the last two seconds," 76ers Coach Billy Cunningham said. "It was a funny game. I never felt we had control, even with a 14-point lead. In Boston when we were up eight (in Game 2), I felt we had control, but today I didn't."

Philadelphia broke the seventh tie of the second quarter and went ahead to stay, 47-45, on Clint Richardson's fast-break layup with 58 seconds left in the first half.

The 76ers increased their advantage to 64-57 midway through the third quarter after Andrew Toney scored twice and Erving made a three-pointer. Following a timeout, Boston came back with five straight points, but Bobby Jones scored six points in a 1:17 span to help the 76ers increase their lead to 74-64 with 2:47 left in the period.

After making 10 of 16 shots and outrebounding the Celtics, 14-7, in the third quarter, the 76ers took an 80-70 lead into the final period. When Toney (16 points) and Erving scored to widen the gap to 14 points, it appeared the 76ers were in command.

"It took us about 30 minutes to adjust our offense after Tiny went down," Fitch said. "It might have taken longer, but we were in the same situation during the regular season."

When Archibald suffered stretched ligaments in his left wrist in mid-February and missed 11 games, the Celtics won 10 of them with Henderson directing the offense.

The Celtics still trailed by 10 (97-87) with 3:26 to play, then outscored the 76ers, 10-2, down the stretch, despite the absence of high-scoring Robert Parish, who was on the bench.

"I took Robert out when we went to our full-court pressure defense," Fitch explained. "We got back in it doing that, so I stayed with the same bunch. I thought we finished on an upbeat."

Maxwell started the final surge with two free throws, then after the 76ers failed to get a shot off in 24 seconds, Kevin McHale made a short hook after getting an offensive rebound. Henderson was fouled on Boston's next possession and made both free throws to reduce the 76ers' lead to 97-93 with 1:51 remaining.

Toney, working against Ainge, made a forced jumper in heavy traffic in the foul circle for the 76ers' first points in two minutes. But Ainge came back with a jumper from deep in the right corner.

Caldwell Jones, who made only three of nine shots after his 22-point performance in Game 2, was called for offensive basket interference on Philadelphia's next possession and Bird brought Boston within two with a 20-footer from the top of the circle with 43 seconds remaining.

After running off almost all of their allotted 24 seconds, the 76ers got Erving a short shot, but he missed and Ainge rebounded with 20 seconds to play. There was no move to call time out and Bird quickly got open, but this wasn't his day.