Liverpool boss Jurgen Klopp is unsure about the extent of Alisson's injury.
The Brazil goalkeeper suffered a hamstring injury at training this week. He was ruled out of today's clash with Brentford.
Klopp said before kickoff: "No I do not know. It's a muscle injury, but we don't know more.
"As always, we have to wait. Maybe we'll know more tomorrow or the day after tomorrow."
Alisson missed the win against Burnley last week due to flu.