London - Liverpool could be thrown out of the League Cup for fielding an ineligible player in their third round victory over MK Dons on Wednesday.
The Athletic reported on Saturday that an international transfer certificate for Pedro Chirivella had not been completed.
Chirivella, who came off the bench in the 63rd minute of the 2-0 win, spent the second half of last season on loan at Spanish club Extremadura.
The 22-year-old, therefore, required new clearance when he returned to his parent club in the summer.
Liverpool requested that in July but the process can only be completed by the English Football Association.
According to the report, the European champions are confident the mitigating factors will ensure they escape the most serious sanction of expulsion from the competition and could instead be fined.
"The club are aware of a potential administrative issue pertaining to one of our players," said a Liverpool statement.
"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."
In a statement, tournament organisers the EFL said there were "currently considering the matter."
Liverpool are due to face Arsenal in the fourth round of the League Cup next month.