The Argentina international is banned after picking up three yellow cards in the 2024-25 tournament, and he is coincidentally also set to serve a domestic suspension in Liverpool's next Premier League match.
Mac Allister should be one of seven missing Liverpool players for the trip to Spain, as Alisson Becker (hamstring), Federico Chiesa (fitness), Kostas Tsimikas (ankle), Ibrahima Konate (knee), Conor Bradley (hamstring) and Diogo Jota (abdominal) remain sidelined.
Slot revealed before Saturday's postponed Merseyside derby that Jota, Chiesa and Alisson were all close to returning, but the trio may target this weekend's clash with Fulham as a more realistic comeback date instead.
With Alisson still out of contention, Caoimhin Kelleher continues to marshal the backline, which should comprise Trent Alexander-Arnold, Andy Robertson, Virgil van Dijk and Konate's deputy Joe Gomez once again.
Mac Allister's ban should see Slot utilise Curtis Jones in a deeper midfield role alongside Ryan Gravenberch, allowing Dominik Szoboszlai to strut his stuff in the most advanced engine room position.
After Darwin Nunez cut a profligate and frustrated figure against Newcastle United in midweek, Slot may consider reintroducing Cody Gakpo into the forward line alongside Mohamed Salah and Luis Diaz, either as the left-winger or centre-forward depending on where he sees fit to field Diaz.
Liverpool possible starting lineup:
Kelleher; Alexander-Arnold, Gomez, Van Dijk, Robertson; Gravenberch, Jones; Salah, Szoboszlai, Diaz; Gakpo
Written by
Ben Knapton