Early this season when the Boston Celtics started running circles around the rest of the National Basketball Association teams and Celtics followers the world over were gushing about their apparent return to the glory days, the players and General Manager Red Auerbach were cautious.

"We barely know each other," Don Chaney said at the time. "I'm afraid to say what might happen by March."

"I don't have time to relish anything because I'm too worried," said Auerbach. "I don't know if we are for real."

Well, March has come and gone and Auerbach and everyone else knows now that the Celtics are real. They finished the regular season with the best record in the league (61-21) and then breezed past the Houston Rockets in the semifinals of the Eastern Conference playoffs.

They are tied at 1-1 with the Philadelphia 76ers, their season-long nemesis, in the best-of-seven conference final series that will be resumed tonight at the Spectrum in Philadelphia.

"Now I can say it's a good ball club," Auerbach said yesterday. "They rose to the occasion all year. They overcame a lot of challenges and won 61 ball games. You can win a series by a fluke, but you can't win 61 games on a fluke. Remember, we had the best road record in the league, too. We're legitimate."

The Celtics also dominated team statistics. Their margin of victory -- 7.8 points a game -- was the best in the league and they were the only team to rank in the top six in both team offense and team defense.

They were also the best three-point field goal shooters in the NBA and the only team to have eight players average in double figures.

The Celtics play in a dingy old building above a train station; they wear those ugly green basketball shoes and they play with a sophomoric enthusiasm that seems out of place in the NBA.

But they are happy, winning and envied.

"We're the team of the past, the team of the present and the team of the future," said M. L. Carr. "We are the team of all teams."

It's not window dressing, either.

In Sunday's 96-90 victory over the 76ers at Boston Garden, Carr who many consider the best sixth man in the game, had played only one scoreless minute as the final seconds neared. When Coach Bill Fitch turned to him to go in, Carr enthusiastically jumped off the bench.

There are too many NBA players who would be embarrassed about going in at that point of the game.

"How can you be embarrassed if you're a Celtic?" Carr asked.

"I don't care the least bit what other people say or think about us," said Auerbach. "We know we're good people and we're not out to hurt anybody. We take care of ourselves and our kind."

There is a long list of valid reasons on how the Celtics turned it around in one season, from a team that was 29-53 a year ago and 32-50 the previous year.

They hired Bill Fitch as coach; they acquired Carr from Detroit; Tiny Archibald was healthy; Dave Cowens became excited about playing again and they got rid of Curtis Rowe and Bob McAdoo.

But the single most important difference is rookie Larry Bird. Seldom has a 6-foot-9, slow player had such an effect on a team -- and the game itself.

Bird can shoot 25-footers, pass, rebound and hustle, but his most impressive skill is one that doesn't show in his individual statistics -- he has the knack for making everyone he plays with better. Such other Celtic greats as Bill Russell and John Havlicek had the knack, too.

"I guess the key thing is that I always know what's happening on the court," Bird said. "I know exactly what I can and cannot do. I know when I can score on a defender. As for my passing, I just see a situation occur and I respond. It's more instinct than anything else.

"Your first year you have to go over and prove you can play to the rest of the league, and help your team win, too. I try to give everything I've got every game. Things have gone real well for me so far. This is a good team for me to be on because no matter how bad someone plays, there is always someone else to pick up the slack."

The Celtics, basically a fast-break team, realize that fast breaks are a luxury in the playoffs, and it becomes a half-court game.

"That's why execution is so important," said Cedric Maxwell, "and we can execute as well as anyone when we have to."

Because they all are such good passers and shooters, the Celtic half-court offense is technically as efficient as their fast break, although it isn't nearly as exciting.

Boston spreads the floor, leaving the middle open for cutters. Four starters -- Cowens, Bird, Chris Ford and Archibald -- are all effective from 18 feet. The other starter, Maxwell, doesn't have much range on his jump shot, but he led the NBA in field goal percentage, making 61 per cent of his shots working close to the basket.

With that front line of 6-9 Cowens, 6-9 Bird and 68 Maxwell, the Celticsscan use any of the three at center or forward and matchups are never a problem for them.