IAMI - The Celtics have noticed a pronounced difference during their last two practices, and it has nothing to do with efforts to reverse a four-game losing streak. The man responsible for the change in attitude at workouts held in American Airlines Arena yesterday and Sunday was not coach Jim O'Brien, but Eric Williams. The forward brought his unique personality, veteran defensive leadership, and energetic style back to the court for the first time since left knee tendinitis sidelined him Nov. 3.