Combine sausage, celery, and onion in a skillet over medium heat. Cook, stirring frequently, until sausage is browned and crumbled, 5 to 7 minutes. Transfer cooked mixture to a plate lined with paper towels to soak up excess oil.
Pour water into a large saucepan and bring to a boil. Stir in butter until melted. Add stuffing mix, cover, and remove from heat. Let stand 5 minutes. Add cooked sausage mixture and stir to combine.