The Merseyside club released a statement on Wednesday confirming the news after scans revealed knee ligament damage. Oxlade-Chamberlain reacted to the news on Twitter:
"Absolutely devastated to have picked up this injury at such a crucial time in the season. Gutted I won't be able to play any further part now in our Champions League run for Liverpool, and also the World Cup with England.
"But now it's all about supporting the boys and getting behind them. I know we have what it takes to do something special in this tournament. I'm going to give everything I have to make sure I'm back as quickly as possible."
He also added his thoughts are with the Reds fan who was attacked ahead of Tuesday's game:
"However, this all pales in comparison to how the family of the Liverpool fan badly hurt before last night's game must be feeling. My thoughts are with him and his loved ones. Thanks for all your kind words, messages and support. I'll see you soon."
Oxlade-Chamberlain went down under a challenge in Liverpool's 5-2 defeat of the Italian side on Tuesday and was stretchered off after 18 minutes.
Liverpool did not confirm exactly how long the 24-year-old will be sidelined but explained the damage is severe enough to rule him out of the rest of the Reds' 2017-18 campaign and the World Cup.
Immediately it is a concern for Liverpool, who will have to face Wednesday's UEFA Champions League semi-final second leg, and likely a final given their first-leg lead, with only three recognised first-team midfielders.
Reds manager Jurgen Klopp has been left with only Jordan Henderson, James Milner and Georginio Wijnaldum to rely upon as Emre Can and Adam Lallana are also occupied on the treatment table.
That might not be such a huge concern were it not for the fact Liverpool have still not guaranteed themselves a top-four Premier League finish this term.
They are eight points ahead of fifth-placed Chelsea but have played a game more than the Blues and also have to travel to Stamford Bridge to take on Antonio Conte's side on May 6.
Meanwhile, England manager Gareth Southgate will likely have been planning to utilise Oxlade-Chamberlain regularly at the World Cup given he has enjoyed a resurgent season after his move from Arsenal to Anfield.