Leon, who had been an unused substitute in his country's draw with Japan, came off the bench for the final nine minutes of a 2-0 defeat to South Korea in Seoul. He replaced Sunderland's Omar Alderete, who played against Ruben Amorim's side in our last Premier League outing.
At only 18 years old, it is a phenomenal achievement for the Reds youngster, who joined the club from Cerro Porteno earlier this year.
By the time the wing-back was introduced, the South Americans already trailed to goals from Swansea City's Ji-sung Eom and Hyeon-gyu Oh of Genk. Diego is still to make his maiden senior appearance for United.