The match in Guadalajara saw South Korea claim a 2-1 victory in their opening World Cup match.
Hyeon-gyu, who now plays for Besiktas, came on as a substitute for legendary captain Son Heung-min in the second half.
The forward, who Brendan Rodgers sold in 2024 to Belgian side Genk, netted the deciding goal after receiving a cross from Feyenoord's Hwang In-beom.
This win follows Mexico's 2-0 defeat of South Africa, which saw three players sent off in the second half.
However, it was a disappointing evening for current Celtic player Yang Hyun-jun.
The 24-year-old was left on the bench for the entire match, with coach Hong Myung-bo opting for other substitutes.
Yang will be hoping for some game-time in South Korea's next match against Mexico in six days' time.
