Bring a large pot of salted water to a boil and cook the manicotti shells according to package instructions, usually about 7 to 9 minutes. Be careful not to overcook them, as they will continue to cook in the oven. Once cooked, drain the shells and rinse under cold water to stop the cooking process. Set aside on a lightly oiled surface to prevent sticking.
In a mixing bowl, combine the shredded chicken, ricotta cheese, mozzarella, parmesan, garlic powder, Italian seasoning, salt, and pepper. Drizzle in the olive oil and mix everything thoroughly until you have a creamy, well-blended filling.
Using a spoon or piping bag, carefully fill each manicotti shell with the chicken and cheese mixture. Take your time to avoid breaking the shells. Place each stuffed shell seam side down in the baking dish.
Pour 1 cup of Alfredo sauce on the bottom of the baking dish to prevent sticking. Arrange the stuffed manicotti shells evenly, then pour the remaining Alfredo sauce over the top, making sure each shell is generously coated.
Cover the baking dish with aluminum foil and bake at 375°F (190°C) for 25 minutes. Remove the foil and bake for an additional 10 minutes to allow the cheese to brown slightly and the sauce to bubble.
Once out of the oven, let the dish cool for a few minutes. Sprinkle with chopped parsley before serving for a fresh pop of color and flavor. Enjoy the creamy, cheesy Chicken Alfredo Stuffed Manicotti while warm!