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 earlier this month.
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 been on holiday in Saint Moritz, a resort town on the Swiss Alps, and missed the Reds' Champions League trip to Girona in midweek.
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 Fulham?
The Argentina international will not be involved in today's game as he is serving a one-match suspension.
Mac Allister reached five Premier League yellow cards against Newcastle, as mentioned.
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And that had earmarked him to be absent against Everton, but that derby was postponed.
As such, his separate one-game ban rolled over to Liverpool's next domestic encounter - which is at home to Fulham today.
He will ultimately have 13 days between games, with the Carabao Cup quarter-final against Southampton on December 18 his next match.
In midweek, he was coincidentally serving a one-match suspension in the Champions League too.
Both of his bans domestically and in Europe were due to an accumulation of yellow cards - but are unrelated.
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 had to serve a one-match ban in Liverpool's next game in the competition.
So he sat out of Liverpool's 1-0 win away at Girona on Tuesday.
Mac Allister's lack of availability for either match this week meant Slot granted him a short break rather than be forced to train.
So, therefore, he's been on holiday over the last week.