Liverpool had hoped that Lallana had only sustained a short-term issue during the Audi Cup final against Atletico Madrid, but it has been revealed that the England international will be absent until October.
Manager Jurgen Klopp told the club’s official website: “This is certainly not news we would have wanted.
“Unfortunately, the damage is such that it will likely mean Adam is out for a couple of months not weeks, so I think we can pretty much rule him out of August and September.
“Beyond that we will monitor and wait and see. He is a boy with a world-class attitude to everything professionally and he has always come back in good time from injury setbacks since I have been here, so we will see.”
As well as the Champions League playoff with Hoffenheim, Lallana will miss Premier League fixtures with the likes of Arsenal and Manchester City.