The Reds attacker will undergo surgery on the injured lateral collateral ligament in his knee according to Colombian journalist Pipe Sierra.
Colombia winger Diaz was forced to return home early from Liverpool's training camp in Dubai to have scans on his knee and will miss Sunday's friendly against Lyon.
Diaz initially suffered the injury against Arsenal in October and only returned to training on Tuesday.
But Klopp had hoped he would return to action for the Reds when they get back to action against Manchester City in the Carabao Cup on December 22.
“It's all good [fitness-wise] with the boys who are here," said Klopp, who has welcome the majority of the squad back from their World Cup break.
“What's not good is we have to see with Lucho [Diaz]. He felt something and we need to have further assessment there, but apart from that they all look really, really good."