On the first day of fall practice the newest Celtic, Willie Naulls, collapsed from exhaustion. "I'm loaded with hustle scars," he said last week. Coach Red Auerbach always starts his teams hard, but this year the regimen was tougher than ever. The Celtics want to get off to a fast start to show that they can win without Bob Cousy and that they can still run and play both ends of the court in spite of an average age of 29, highest in the league. In Boston the name the players give their game is "big D," the D for defense. The man who spells the whole word out is Bill Russell, still magnificent and even more determined. His major cohorts are Tom Sanders, an excellent defensive corner man, and K. C. Jones, a guard who is such a master of harassment that he often keeps opponents from even getting the ball. K. C. is Cousy's replacement, but it is Naulls (12.9) who should make up the scoring lost when Cousy (13.2) retired. Bought from San Francisco, Naulls is the latest of the veteran replacements that the Celtics always seem to come up with. The only other Celtic newcomers are also experienced. They are Guards Johnny McCarthy (ex- St. Louis) and Larry Siegfried (ex-ABL). Sam Jones (19.7) and Tommy Heinsohn (18.7) will continue to lead the scoring, though Heinsohn is slowed for now because of shingles. John Havlicek, last year's fine rookie, is now more of an outside shooting threat than he was. Havlicek is another Auerbach swing man. He and Frank Ramsey can play both forward and guard, providing a versatility that makes Boston even deeper. The Celtics will need depth for their fight with the Royals. " Cincinnati is a helluva team," Auerbach says. But everybody knows Boston is, too.