In a large mixing bowl, combine Greek yogurt, Dijon mustard, lemon juice, garlic powder, salt, and black pepper. Whisk until the dressing is homogeneous.

Add the shredded chicken, diced celery, minced red onion, and chopped parsley to the bowl.

Using a spatula, fold the ingredients together until the chicken is thoroughly and evenly coated with the yogurt dressing.

Taste and adjust seasoning with additional salt or pepper if required.

Transfer to an airtight container and refrigerate for at least 30 minutes to allow the flavors to meld before serving.