In a large saucepan, saute the onion, celery and garlic in butter until tender. Stir in the salsa, ketchup, brown sugar, vinegar, Worcestershire sauce, chili powder, salt and pepper. , Stir in chicken. Bring to a boil. Reduce heat; cover and simmer for 15 minutes. Serve about 1/3 cup chicken mixture on each bun. Freeze option: Freeze cooled meat mixture in freezer containers. To use, partially thaw in refrigerator overnight. Heat through in a saucepan, stirring occasionally and adding a little water if necessary. Serve in buns.