It was one of the classic mismatches in recent NBA playoff history, the sort of defensive duty that coaches avoid assigning if at all possible. It was a seldom-used rookie against one of the most versatile offensive players in the league.

This confrontation was, well, a big reason why the Boston Celtics, the defending league champions, are down, 3-1, and on the edge of elimination in the Eastern Conference's final series. The fifth game against the Philadelphia 76ers will be played tonight at 8 o'clock in sold-out Boston Garden. The game will be shown in the Washington area on the USA cable network.

The mismatch was the Celtics' Danny Ainge against Andrew Toney, Philadelphia's lightning-quick, 6-foot-3 guard. The nightmare that Boston Coach Bill Fitch has been living with is that it probably can't be avoided again.

For the second year in a row, the Celtics are two games down to Philadelphia in the conference final, largely because of their inability to contain Toney. Last year, he scored 26 points in the opener and 35 in the second game before leveling off to a 20-point average when the Celtics rallied to win the last three games.

During the regular season, Toney made 54 percent of his shots and averaged 25.2 points against Boston. In the 76ers' three playoff victories, he's scored 30, 16 and 39 points.

"I don't know what I can do differently to stop him," Ainge said Sunday after the fourth game as he sat alone in a corner of the Celtics' dressing room, holding an ice cube on a cut lip that required six stitches.

"I've never had an experience like that before," the rookie admitted when asked about a six-minute stretch when he committed five fouls, was called for his first technical and Toney scored 13 points.

"This was only the second time I guarded him all year," Ainge went on, softly. "The guy was just unstoppable. You knew he was hot because he was looking for the ball all the time. I had a hand in his face. I tried to keep the ball away from him and, once he got it, I tried to get up on him to make hm drive so I could get help from our big men."

Ainge knows all the proper defensive techniques. He's an intelligent player and he has size and quickness. But in basketball, at any level, there are times when one player simply doesn't have the physical ability to stop another one on one.

Ainge is Fitch's last hope. The Celtics have been starting M.L. Carr at the guard spot opposite playmaker Gerald Henderson, who matches up well against Philadelphia's Maurice Cheeks. But Carr doesn't have the quickness to stay with Toney and neither does his replacement, Chris Ford. That leaves Ainge.

"We've got to change our defensive strategy," Ford said. "We can't allow Toney to have all those shots, to be the one to beat us. Denying him the ball is real tough. You've got to double-team him, triple-team him and, if we're going to get beat, get beat by someone else."

Larry Bird, who also was asked to guard Toney, agreed with Ford and probably tipped off tonight's game plan.

"We can't play a straight defense, one-on-one against Toney," the all-star forward said. "We've going to have to double up and see if Bobby Jones or Caldwell Jones can beat us."

What makes the 6-foot-3 Toney so difficult to contain is his versatility. He's an excellent outside shooter and also has the ability to drive and draw fouls. Despite averaging only 32 minutes a game in this series, he's shot the most free throws on his team (20 of 25).

Jack McMahon, the 76er scout who recommended both Cheeks and Toney "because they both have the ability to take the ball to the basket," is one of Toney's biggest admirers.

"They talk about running to daylight in football," McMahon said. "That's what Toney does. It doesn't matter where he is when he receives the ball. And he's just relentless offensively. You can't discourage him. The most important thing in the playoffs is to have a player who can get his shot."

Coach Billy Cunningham likes to think he has a lot more in his offensive arsenal. He can call on Julius Erving, who is averaging 17 points a game in this series, Cheeks and, probably Darryl Dawkins, who has contributed 28 points the last two games as a reserve. However, if it comes down to one shot tonight, the odds are Toney will take it.