Season chicken, if desired, with salt and pepper. Melt margarine in large nonstick skillet over medium-high heat and cook chicken, stirring occasionally, until chicken is thoroughly cooked, about 5 minutes. Remove chicken and set aside.
Add mushrooms and squash to same skillet and cook, stirring occasionally, until squash is almost tender, about 5 minutes. Add Knorr® Sidekicks® Harvest Chicken Rice & Vermicelli Side Dish and water and bring to a boil over high heat. Reduce heat and simmer covered, stirring occasionally, until rice is tender, about 7 minutes. Stir in chicken; heat through.