Preheat your oven to 350°F (175°C).
In a large skillet, heat the olive oil over medium heat. Add the chicken pieces and season with garlic powder, onion powder, paprika, salt, and pepper. Cook until the chicken is browned and cooked through, about 6-8 minutes.
Stir in the barbecue sauce and cook for an additional 2-3 minutes until heated through.
In a large mixing bowl, combine the cooked spaghetti, chicken mixture, diced tomatoes, and half of the shredded Monterey Jack cheese. Mix well until everything is evenly coated.
Transfer the mixture to a greased 9x13 inch baking dish. Top with the remaining Monterey Jack cheese and sliced green onions.
Bake in the preheated oven for 20-25 minutes, or until the cheese is melted and bubbly.
If desired, garnish with chopped cilantro before serving.