The Senegal global picked up the problem during Senegal's 2-0 victory over Cape Verde on Saturday evening and was replaced in the 89th minute of the World Cup qualifier.
The Reds released a statement on their website on Tuesday evening confirming that Mane had suffered a hamstring problem and would be out for the next six weeks.
Liverpool, now sitting seventh in the Premier League, can ill-afford to drop points against the rest of the "big six" in such a tight race for the Champions League places, never mind the Premier League title itself.
The visitors are so far unbeaten in the Premier League, having drawn just one and won six of their seven matches.
What do Liverpool lose without Mane?
Mohamed Salah has been brilliant since arriving at Liverpool, and hot on the heels of scoring the goal that sent Egypt to the World Cup, he's claimed that the club can and will cope without Sadio Mane. Since making the move from Southampton, Liverpool have averaged 2.2 goals a game with Mane in the lineup, compared to 1.6 goals without him.
Who will cover for Mane?
"You would have never said before Mane joined he'd be the most pivotal player but you would now say, given what happened during the African Cup of Nations last season, losing him is a big psychological blow".
The United game is obviously huge for Liverpool, but legend Kenny Dalglish claims that Klopp needs to be backed by the club no matter what the result, and that winning the title will take time and transfer windows for Klopp to get the side he wants.