The identity of the player is unknown but Jürgen Klopp fielded several youngsters in Wednesday’s 2-0 win, starting three debutants in Rhian Brewster, Harvey Elliott and Caoimhin Kelleher alongside teenagers Ki-Jana Hoever and Curtis Jones.
Sepp van den Berg and Herbie Kane made their first senior appearances for the club from the substitutes’ bench, while 22-year-old midfielder Pedro Chirivella played for the first time in three years.
A Liverpool spokesperson said: “The club are aware of a potential administrative issue pertaining to one of our players.
“We are working with the relevant authorities in order to establish the facts of the matter and will be making no further comment until this process is complete.”
An EFL spokesperson told The Independent: "We are currently considering the matter."
EFL Cup rules state that any players from Premier League clubs must be ones "registered and eligible to participate in a Premier League match commencing at the same time and on the same date as the competition match".
The EFL warns clubs that it has "full power" to investigate clubs in breach of the competition's rules and impose “penalties by way of reprimand, fine or other penalty (or any combination thereof)”.