What has happened?
The pair have both been called up to represent Brazil during the current international break. Roberto Firmino has not been named in the squad after recently returning from a hamstring injury.
Alisson and Fabinho missed Brazil's fixtures last month's because of quarantine rules.
With the South American country on England's Covid-19 travel red list, they would have had to self-isolate in a mandatory hotel for 10 days.
The government has since relaxed rules for footballers who have been double vaccinated.
They can now depart their 10-day isolation in "bespoke quarantine facilities" once a day for training or to play matches.
Why will they miss the Watford clash?
It's looking highly likely that Alisson and Fabinho will both be absent for the trip to Watford on Saturday 16 October in the Premier League. Kick-off is at 12.30 BST.
Brazil face Uruguay on Friday, October 15, with the game starting at 01.30 in the UK.
That means they won't return in time for the Vicarage Road fixture.
There is the option of Brazil releasing the duo early after matches against Venezuela (October 10) and Colombia (October 12).
However, according to Globo Esporte, head coach Tite is not prepared to do so after having such a depleted squad last month.
Liverpool reportedly requested kick-off for the Watford game was pushed back until 19.45 but were denied.
Why could they miss the Atletico game?
Liverpool travel to the La Liga champions on Tuesday 19 October (20.00 BST).
At that point, Alisson and Fabinho will still be in isolation after returning to England. Therefore, they may not be allowed out of quarantine to travel to Spain.
When might they be back available?
Alisson and Fabinho should fine for Liverpool’s trip to Manchester United on Sunday 24 October as long as they don’t suffer injury.
Could Liverpool block Alisson and Fabinho from going on international duty?
No. Liverpool could face sanctions should Brazil invoke FIFA rule to prevent them from appearing.
Last month, the Reds expected to be without Alisson, Fabinho and Firimino for five days after the international break before the rules were waived.
Who will replace the pair?
In the absence of Alisson, young keeper Caoimhin Kelleher will get a rare opportunity.
The Republic of Ireland international has made just one appearance so far this season in the 3-0 win over Norwich in the Carabao Cup.
The task of shielding Liverpool's defence without Fabinho may fall on the shoulders of captain Jordan Henderson.
The versatile James Milner is also an option if right-back Trent Alexander-Arnold returns to fitness.