Preheat the oven to 350ºF and spray a 9-inch square baking dish with nonstick spray.
Bring a large pot of salted water to a boil, cook the spaghetti according to package directions until al dente, drain and set aside.
Cut the chicken into bite-size pieces and toss with the taco seasoning until evenly coated.
In a large skillet, melt the butter over medium-high heat. Add the seasoned chicken, garlic powder, and onion powder, and cook until the chicken is cooked through and no liquid remains in the skillet, about 6–8 minutes.
Reduce heat to low and stir in the cream of chicken soup, undrained Rotel tomatoes, and cubed Velveeta. Cook, stirring constantly, until the Velveeta has melted and the mixture is smooth.
Stir the cooked spaghetti into the cheese and chicken mixture until evenly combined, then transfer the mixture to the prepared baking dish.
Sprinkle the shredded cheddar cheese evenly over the top and bake for 25–30 minutes, or until heated through and the cheese is melted.