"This is certainly not news we would have wanted," Klopp told the club website (www.liverpoolfc.com).
"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."
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Lallana joined Liverpool from Southampton in July 2014 and scored eight league goals last season.
He will now miss the start of Liverpool's next Premier League campaign, which begins at Watford on Aug. 12, three days before the first leg of their Champions League playoff against Hoffenheim.
He will also be a major doubt for England's next two World Cup qualifiers, against Malta and Slovakia in September.
(Reporting by Hardik Vyas in Bengaluru, editing by Julian Shea)
