Combine chicken broth, celery, onion, and butter into a 2-quart saucepan with a tight-fitting lid.

Bring the mixture to a boil over medium-high heat.

Add the rice to the boiling broth mixture.

Reduce heat to low and cover the saucepan tightly.

Simmer for approximately 20 minutes, or until rice is partially cooked.

Add the chopped cooked chicken, salt, and pepper to the saucepan.

Continue to simmer, covered, for about 10 more minutes.

Check if the rice is fluffy and the liquid has been absorbed.

If needed, simmer for a few more minutes until rice is fully cooked and fluffy.