Preheat the oven to 350°F.
Cook the manicotti shells according to package instructions until al dente. Drain and set aside to cool slightly.
In a mixing bowl, combine the shredded chicken, ricotta cheese, half of the mozzarella cheese, half of the Parmesan cheese, garlic powder, Italian seasoning, salt, and pepper. Mix until well combined.
Stuff each manicotti shell with the chicken and cheese mixture.
Spread 1 cup of Alfredo sauce in the bottom of a 9x13 inch baking dish.
Arrange the stuffed manicotti on top of the sauce.
Pour the remaining Alfredo sauce over the manicotti, ensuring they are well covered.
Sprinkle the remaining mozzarella and Parmesan cheese on top.
Cover the dish with aluminum foil and bake for 25 minutes.
Remove the foil and bake for an additional 10-15 minutes, or until the cheese is bubbly and golden.
Let the dish cool for 5 minutes before serving. Garnish with chopped parsley.