Preheat the oven to 350°F and spray a 9x9-inch baking dish with nonstick spray; set aside.
Cook the spaghetti in a pot of boiling salted water until al dente, drain well, and set aside.
In a large saucepan over medium heat, add the cubed Velveeta, cubed cream cheese, undrained Rotel tomatoes, and cream of chicken soup.
Add the minced garlic and Italian seasoning to the saucepan and cook, stirring frequently, until all the cheeses melt and the sauce is smooth.
Stir the shredded chicken and cooked spaghetti into the cheese sauce, mixing gently until the noodles and chicken are evenly coated.
Transfer the mixture to the prepared baking dish and sprinkle the shredded mozzarella evenly over the top.
Bake for 20–25 minutes, or until the casserole is heated through and the cheese on top is melted.
Remove from the oven and let rest briefly before serving; serve with optional red pepper flakes or garlic bread if desired.