The World Cup-winning Argentine was due to serve a one-match ban against Everton before the Merseyside derby was postponed.
Mac Allister received his fifth yellow card of the season in the Premier League during Liverpool's 3-3 draw with Newcastle on Wednesday.
As a result, the former Brighton star was set to be sidelined for his new club's short trip to Goodison Park last weekend.
However, the first Merseyside derby of the campaign was called off hours before kick-off due to adverse weather.
The rescheduled showdown is not expected until early 2025 due to both Liverpool and Everton's jam-packed run of festive fixtures.
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Mac Allister meanwhile has already headed on holiday to Saint Moritz, a resort town on the Swiss Alps, and won't play at all this week.
The 25-year-old has been a constant figure under Arne Slot this season but has now left the Reds short at a vital time.
Why is Mac Allister not playing against Girona?
The Argentina international is coincidentally serving a one-match suspension in both the Champions League and the Premier League.
Both of his bans domestically and in Europe are due to an accumulation of yellow cards - but are unrelated.
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Mac Allister picked up this third Champions League booking in the 2-0 win over Real Madrid, a match he later scored in.
As per UEFA rules, the £35million signing has to serve a one-match ban in Liverpool's next game in the competition.
Why is Mac Allister also banned against Fulham?
His Champions League suspension just so happens to have fallen adjacent to a domestic match that he was due to be sidelined for.
The bookings threshold is lower in European competition due to the reduced number of games, with bans not carrying over.
Mac Allister's fifth Premier League caution had earmarked him to be absent against Everton, but that derby was postponed.
As such, his separate one-game ban rolls over to Liverpool's next domestic encounter - which is at home to Fulham on Saturday.
Mac Allister's lack of availability for either match this week meant Slot granted him a short break rather than be forced to train.
He will ultimately have 13 days between games, with the Carabao Cup quarter-final against Southampton on December 18 his next match.
Girona vs Liverpool: How to follow
This Champions League clash is set to take place on Tuesday, December 10.
Kick-off at Estadi Montilivi in Spain is scheduled for 5:45pm UK time.
talkSPORT 2 will have live coverage from Catalonia, with commentary from Adam Bridge and former Watford striker Troy Deeney.
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talkSPORT.com will also have updates in our dedicated live Champions League blog.
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