Preheat the oven to 350°F (175°C).
Cook the spaghetti in a pot of salted boiling water 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 the minced garlic, and sauté 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 the undrained Rotel; stir constantly until the cheeses melt and the sauce is smooth.
Stir the drained spaghetti into the cheese and chicken mixture until the pasta is fully coated; season with salt and black pepper to taste.
Lightly grease a 2‑quart casserole dish, transfer the cheesy chicken spaghetti into it, and sprinkle the grated cheddar cheese evenly over the top.
Bake in the preheated oven 25–30 minutes, until heated through and the cheese on top is melted.
Let the casserole rest a few minutes, then serve.