In a medium bowl, add the chicken and buffalo sauce. Stir to combine.
Add the celery, red onion, salt, and pepper. Stir to combine and set aside.
Make the Greek yogurt ranch dressing: in a small bowl, add the Greek yogurt, dried parsley, garlic powder, dried dill, onion powder, salt, pepper, and lemon juice. Stir to combine.
Lay each lettuce leaf flat on a plate and scoop the chicken mixture into the middle. Top with a spoonful of Greek yogurt ranch dressing. Garnish with blue cheese crumbles and sliced green onion, if desired.