This Is Anfield understands that the Spaniard has not been included in the Reds selection for the first part of the competition.
Differences between UEFA and Premier League rules meant Jurgen Klopp was forced into a difficult decision over the composition of his squad for Europe this season.
Despite neither possessing homegrown status, Ryan Gravenberch and Ben Doaks ages mean they are both able to be registered as U21 players when it comes to the league this term.
But that is not the case in Europe, where both are only eligible to feature alongside other foreign stars on List A.
That left Klopp with 18 players and only 17 spots in which to register them, and it is Adrian who has been forced to miss out as a result.
As third-choice goalkeeper behind Alisson Becker and Caoimhin Kelleher, the 36-year-olds chances of featuring in the Europa League this season appeared slim.
However, his omission is also understood to be down to the presence of a UEFA loophole that could still see him feature in some capacity.
The rules state that, in the case of any named goalkeepers being struck down by long-term injury or illness, a replacement can be called upon.
Therefore, if the Reds options between the sticks were thinned by the loss of either Alisson or Kelleher, they would be able to utilise Adrian in Europe.