Spray the crock of a slow cooker with nonstick cooking spray. Place chicken breasts in the crock. Combine water and Italian dressing mix in a small bowl. Pour over chicken; sprinkle with garlic, cover, and cook on Low for 4 hours.

After 4 hours, whisk cream cheese and condensed soup together in a bowl. Pour over chicken. Stir in mushrooms. Cover and cook on Low for 1 hour.

Fill a large pot with lightly salted water and bring to a rolling boil over high heat. Stir in spaghetti, and return to a boil. Cook pasta uncovered, stirring occasionally, until cooked through but still firm to the bite, about 12 minutes. Drain well.

To serve, spoon chicken and sauce over hot cooked pasta and sprinkle with parsley.