In a medium bowl, mix together the plain fat-free Greek yogurt, mayonnaise, and fresh lemon juice. Stir in the green onions and minced parsley, seasoning with salt and pepper to taste.

In a large mixing bowl, combine the diced chicken, bacon, grape tomatoes, and celery if using. Gentle mix to distribute ingredients evenly.

Pour the dressing over the salad mixture and gently toss everything together until thoroughly coated.

Carefully fold in the diced avocado, combining gently to avoid mashing.

Serve by placing portions of the chicken salad mixture into the romaine lettuce leaves, wrapping them as desired.