Preheat the oven to 350°F (175°C) and spray a 9×9-inch baking dish with nonstick spray; set aside.
Boil the spaghetti in salted water until al dente, drain well in a colander, and set aside.
In a large saucepan over medium heat, combine cubed Velveeta, cubed cream cheese, Rotel tomatoes (undrained), and cream of chicken soup.
Add the minced garlic and Italian seasoning to the saucepan and cook, stirring frequently, until all cheeses melt and the sauce is smooth and heated through.
Stir the shredded chicken into the sauce, then add the drained spaghetti and toss gently to coat all noodles evenly.
Transfer the mixture to the prepared baking dish and sprinkle the shredded mozzarella evenly over the top.
Bake for 20–25 minutes, until heated through and the cheese on top is melted.
Let the casserole sit a few minutes before serving; serve hot.