1. Stir chicken, bacon, tomato, and thinly sliced celery together in a bowl.
2. Whisk mayonnaise, parsley, green onions, lemon juice, Worcestershire sauce, salt, and black pepper together in a bowl until dressing is smooth. Pour dressing over chicken mixture; toss to coat. Refrigerate until chilled, at least 30 minutes.
3. Stir chicken mixture and serve over romaine lettuce leaves; garnish with avocado slices.