The following year, he was named as the National League MVP as Ulsan finished top of the division.
Midway through the 2019 season Koo left to complete his mandatory military service with Yangju Citizen FC in the K League 3 before returning to Busan in April 2021.
In 2023, Koo played every minute of every game for Busan as they finished second in the league, with the club missing out on promotion after a 4-6 two-leg aggregate defeat to Suwon FC in the play-offs.
Following the 2023 season, only goalkeeper Kim Poong-joo has kept more clean sheets for Busan in the club's history.
This biographical article related to a South Korean association football midfielder is a stub.