Rub the garlic powder over the raw chicken; and cut it into 1-in. strips.
Stuff the chicken into the raw manicotti shells.
Spread 1 cup spaghetti sauce in each of two greased 13-in. x 9-in. baking dishes.
Place eight stuffed manicotti shells in each dish.
Sprinkle with sausage and mushrooms. Pour remaining spaghetti sauce over the top. Sprinkle with cheese.
Drizzle water around the edge of each dish. Cover and bake one casserole at 375° for 65-70 minutes or until chicken is no longer pink and pasta is tender. Cover and freeze remaining casserole for up to 1 month.
To use frozen casserole: Thaw in the refrigerator. Let stand at room temperature for 30 minutes. Bake as directed.