Rookie girl group member Nayoon of Good Day has suffered a ligament rupture, according to her label C9 Entertainment on November 1. 

"Hello. This is C9 Entertainment.
Thank you to all of you for always loving and supporting Good Day wholeheartedly. Today, we deliver unfortunate news. 

Toward the end of October, Good Day member Nayoon suffered an injury where she ruptured a ligament during practice, resulting in surgery. 

Currently, her leg is in a full cast, and it is unavoidable that Good Day now promote as 9 members until her ligament reattaches completely. According to the doctor, the injury will take approximately 8 weeks to recover, and afterward, [Nayoon] plans on receiving physical therapy in order to return to the stage. 
We at C9 Entertainment will fully support Nayoon so that she can recover wholly, and will work hard so that fans can see her bright smile on stage again as quickly as possible. 

We ask fans for understanding as well as support for the 9 member Good Day. Thank you."