Preheat the oven to 350°F (177°C). Lightly grease a 2-quart casserole dish and set aside.
Cook the spaghetti according to package directions until al dente, drain, and set aside.
Melt the butter in a large skillet over medium to medium-high heat.
Add the bite-sized chicken pieces to the skillet, sprinkle with onion powder, add minced garlic, and sauté about 7–10 minutes until the chicken is cooked through and no longer pink.
Reduce heat to low and add the cream of chicken soup, cubed Velveeta, cubed cream cheese, and undrained Rotel to the skillet.
Stir constantly over low heat until the cheeses melt and the mixture is smooth and combined.
Add the drained spaghetti to the skillet and stir until the pasta is fully coated with the cheese and chicken mixture. Season with salt and black pepper to taste.
Transfer the mixture to the prepared casserole dish, sprinkle the grated cheddar evenly over the top, and bake for 25–30 minutes until heated through and the cheese is melted.
Remove from oven, let rest briefly, then serve.
