Spray the crock of a slow cooker with non-stick cooking spray. Place chicken breasts in crock. Combine Italian dressing mix, and water in a small bowl. Pour over chicken; sprinkle with garlic, cover, and cook on Low 4 hours.
After 4 hours, whisk softened cream cheese and cream of chicken soup together in a bowl. Pour over chicken; stir in mushrooms. Cover and cook on Low for 1 additional hour.
Fill a large pot with lightly salted water and bring to a rolling boil over high heat. Once the water is boiling, stir in the spaghetti, and return to a boil. Cook the pasta uncovered, stirring occasionally, until the pasta has cooked through, but is still firm to the bite, about 12 minutes. Drain well in a colander set in the sink.
To serve, spoon chicken and sauce over hot cooked pasta and sprinkle with parsley.