Pedro Chirivella has been identified as the player at the centre of an EFL investigation that could see Liverpool booted out of the Carabao Cup.
It is thought the 22-year-old did not have the necessary international clearance to play in the match. He came on as a 63rd minute substitute for Naby Keita.
It is believed he needed fresh international clearance to play for the Reds but they made an application in July.
The Premier League leaders are said to be confident they will not be kicked out of the cup as they feel it is an administrative error from the FA.
They are due to face Arsenal in the fourth round next month.
A Reds spokesman confirmed: “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.”
Liverpool signed Chirivella in 2013 from Valencia and went on to make his debut for the club two years later when Brendan Rodgers was the manager.
His appearance against MK Dons was the first time he had played for the club since May 2016.
He had been on loan to Dutch sides Go Ahead Eagles and Willem II before joining Extremadura in January.