Preheat the oven to 350°F (175°C) and grease a 9x13-inch baking dish.
In a large bowl, beat the softened cream cheese until smooth, then fold in the thawed, well-drained spinach and garlic powder until evenly combined.
Stir in the diced cooked chicken and sliced green onions, then taste and adjust seasoning with salt and black pepper as needed.
Stuff the chicken and spinach mixture into the cooked manicotti shells and arrange them in the prepared baking dish.
In a large saucepan over medium heat, melt the butter. Whisk in the flour and cook, stirring, for about 2 minutes to form a roux.
Add the salt, black pepper, Italian seasoning, and crushed red pepper flakes to the roux, then gradually whisk in the milk until smooth.
Bring the sauce to a gentle simmer and cook, whisking frequently, until it thickens slightly.
Remove the sauce from heat and stir in the shredded Parmesan until melted and smooth, then pour the sauce evenly over the stuffed manicotti.
Sprinkle a little extra Italian seasoning on top if desired, then bake uncovered for 30 to 35 minutes until bubbly and heated through.