Liverpool centre-back Virgil van Dijk needs surgery on a knee ligament injury sustained in the 2-2 draw at Everton on Saturday.
The 29-year-old Netherlands defender was unable to continue following a rash challenge by Toffees keeper Jordan Pickford in the first half.
It is unclear how long he will be out for with the anterior cruciate ligament (ACL) injury.
He went for a scan before a specialist confirmed the extent of the damage.
Returning to action after ACL surgery can take many months and Liverpool will not specify how long the Dutchman will be out for until after his operation.
Surgery will not be immediate and the operation has yet to be arranged.
Club sources are reluctant to rule out a return to action this season, as much will depend on his rehabilitation.
Van Dijk has been a key figure for Liverpool since signing for them in January 2018 for £75m.
He has played every league game for the club in the past two seasons as Liverpool finished second in 2018-19 before winning their first top-flight title in 30 years in 2019-20.
The Dutchman was also a leading figure for the side as they won the Champions League in 2019 and Club World Cup later that year.
He had played seven times for the Reds this season prior to the Merseyside derby.