Last year the Celtics could have won in a walk, and they almost did, beating Philadelphia by 11 games for their fifth consecutive Eastern Division title. This year they are still the best in the NBA, but they will have to run all the way to prove it. Bill Sharman (16.3 points a game) and Gene Conley (substitute center) are gone. Bob Cousy is still matchless, but he is also 33. Coach Red Auerbach has nine returning veterans, however, so the changes won't be fatal. The Joneses (Sam and K.C.), line-drive-shooting Tom Heinsohn, Frank Ramsey and Tom Sanders, who performed so well as a rookie in last year's playoffs, will share Sharman's points. Cousy paces himself well, and Auerbach can afford to rest him often. Bill Russell, the defensive wonder who was last season's MVP, is back with a goatee and a better shot. The Celtics' future depends on Russell. If he stays healthy, it's Boston.