Melt butter in a large pot over medium-high heat. Add carrots, celery and onion and sauté for 5 minutes. Add garlic and sauté for an additional minute.
Sprinkle flour on top of vegetables and cook for 2-3 minutes, or until flour begins to turn golden brown.
Stir in chicken broth and bring to a boil. Reduce heat to medium-low and let simmer for 15 minutes. Add orzo and cook for 10 minutes, stirring frequently to make sure it doesn’t stick to the bottom of the pot.
Add cooked chicken, lemon juice, Italian seasoning, salt and pepper and let simmer for a few minutes, until heated through, and serve.