Complex sentence: In the first round of the NBA playoffs against the Charlotte Hornets the Celtics were stunned by the loss of Lewis, their leading scorer, who collapsed on the court during Game 1 and was diagnosed with what eventually proved to be a fatal heart condition.
Split into simpler sentences: In the first round of the NBA playoffs against the Charlotte Hornets the Celtics were stunned by the loss of Lewis, their leading scorer. He collapsed on the court during Game 1 and was diagnosed with what eventually proved to be a fatal heart condition.