Preheat the oven to 350°F (175°C).
Cook the spaghetti in a pot of boiling salted water according to package directions until al dente, then drain and set aside.
Heat 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, and add the minced garlic; sauté 7–10 minutes until the chicken is cooked through and no longer pink.
Reduce heat to low and stir in the cream of chicken soup, cubed Velveeta, cubed cream cheese, and undrained Rotel; cook, stirring constantly, until the cheeses melt and the sauce is smooth.
Stir the drained spaghetti into the cheese and chicken mixture until evenly coated; season with salt and black pepper to taste.
Lightly grease a 2-quart casserole dish, transfer the cheesy chicken spaghetti to the dish, and sprinkle the grated cheddar evenly on top.
Bake in the preheated oven for 25–30 minutes, until heated through and the cheese on top is melted.
Let the casserole rest a few minutes, then serve.