Jurgen Klopp says Luis Diaz's injury setback is "a proper smash in the face" as the Liverpool winger awaits news on the full extent of the damage.
Diaz injured his knee during a training session at Liverpool's base in the United Arab Emirates and flew back to the United Kingdom on Friday to undergo tests.
The Colombia international was on the comeback trail after missing 10 games for the Reds prior to the World Cup hiatus due to an injury sustained against Arsenal on October 9.
Klopp remains unsure for how long Diaz will be on the sidelines, but accepts it is a blow to lose the 25-year-old ahead of Liverpool's campaign resuming in 11 days' time.
"He was not in the best possible place. It's clear it's a big disappointment for all of us, for him as well," Klopp told reporters after Sunday's 3-1 friendly defeat to Lyon in Dubai.
"It was a non-situation in training. Honestly – nothing. He felt something. He didn't feel a lot the next day, but we wanted to be really cautious and said, 'OK, come on, let's have a look'.
"Then the news came and it was a proper smash in the face. But that's it now."
Former Porto attacker Diaz has four goals and two assists in 12 games this season, with 11 of those appearances being starts.
He has been directly involved in 16 goals in 38 games since his debut in February – a tally only Diogo Jota (17), Roberto Firmino (18) and Mohamed Salah (33) can better.
Diaz is now in a race against time to be fit for Liverpool's next competitive match, which is a mouth-watering EFL Cup last-16 tie with Manchester City on December 22.
Klopp provided some better injury news regarding midfielder Harvey Elliott, who limped off after 29 minutes of the Dubai Super Cup opener with Lyon.
"Harvey got a knock before we scored, but I think we were lucky," Klopp said, quoted on Liverpool's official website. "He looks fine but he feels it. That's normal; there was contact. I hope we were lucky in that moment."
Joao Carvalho gave Liverpool the lead inside a minute against Lyon, but Alexandre Lacazette scored twice, either side of a Bradley Barcola goal, to earn the French side victory.
Liverpool, who are sixth in the Premier League with 14 matches played, face Milan on Friday in the second of their mid-season friendlies.
