Preheat the oven to 350°F (175°C).
In a large mixing bowl, combine the shredded chicken, buffalo sauce, cooked rice, and cream cheese. Mix until fully combined.
Add in the diced celery, green onions, garlic powder, salt, and pepper. Stir until everything is evenly distributed.
Pour the mixture into a greased 9x13 inch casserole dish, spreading it evenly.
Top the mixture with shredded cheddar cheese, spreading it evenly over the surface.
Bake in the preheated oven for about 25-30 minutes, or until heated through and cheese is bubbly.
Let the casserole cool for 5-10 minutes before serving.