The Brazilian international missed last week’s win over Burnley with an illness and picked up the injury during Friday’s final training session.
This injury is a massive blow for Liverpool manager Jurgen Klopp, who was happy to have his number one back in goal just hours earlier.
Back-up keeper Caoimhin Kelleher is already confirmed to start next week’s League Cup final against Chelsea. He will now be called up for the Brentford fixture.
Alisson’s injury only adds to Liverpool’s growing concerns, with Trent Alexander-Arnold out for the next few weeks and Dominik Szoboszlai a major doubt for the trip to Wembley Stadium.
Thiago Alcantara returned from an injury that ruled him out for almost a year, only to suffer another setback, adding to the team’s woes.
Despite the growing injury list, Klopp defended his medical department against accusations of rushing players back too soon.
He explained that player safety is paramount and that final decisions are always made in consultation with the player and medical staff.
There is a glimmer of hope with Mohamed Salah returning to the squad after recovering from a hamstring issue.
The Egyptian has been sidelined since he returned from the Africa Cup of Nations but returned to give the Reds a major boost heading into the final stretch of the season.
Klopp will be desperate to get his key players back before Manchester City’s visit on March 10.
This match could be a title decider, and he will need all hands on deck as he looks to bow out with Premier League honours.