Jurgen Klopp, the Liverpool manager, confirmed after his side's humbling at Villa Park that the Brazil international would be sidelined for a lengthy amount of time.
Alisson suffered the injury in after clashing with a team-mate in training and was replaced in the starting line-up by back-up goalkeeper Adrian, who had a night to forget in the west Midlands.
Liverpool return to Premier League action with a Merseyside derby against Everton at Goodison Park after the forthcoming international break, but Klopp said Alisson had "no chance" of recovering in time to play.
"It could have been worse, for sure," he said of the 28-year-old's injury. "I don't want to put a timeframe but four weeks is possible, six weeks more likely."
Alisson missed seven games at the start of last season with a calf injury, forcing Klopp to rely on deputy Adrian. Further injuries and suspensions to Alisson saw Adrian make 11 league appearances in total.
"Adrian is a really good goalie," Klopp said. "He played 11 games last year, we won all of them. Tonight our goalie was not the problem. The reaction to that first goal was not his problem, it was our problem."
A six week lay-off would see Alisson unavailable to play in the first half of Liverpool’s Champions League group stage campaign, missing the meetings with Ajax, Midtjylland and Atalanta.