Virgil van Dijk has undergone knee surgery after he damaged ligaments in Liverpool's Merseyside derby draw against Everton.
Van Dijk was injured early in the 2-2 draw when he was at the receiving end of a reckless challenge by Everton goalkeeper Jordan Pickford, which left Liverpool manager Jurgen Klopp fuming.
"The Liverpool defender's planned operation took place in London and was conducted successfully," the club said in a statement. "Van Dijk will now immediately focus on the beginning of his rehabilitation with the support of the Reds' medical department."
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The club did not give a time-frame on his return, with the Dutch defender potentially ruled out for the rest of the season due to the nature of the injury.
Midfielder Fabinho, who has been slotting into the back four in Van Dijk's absence, suffered a hamstring injury against Midtjylland on Tuesday and is unavailable. Rhys Williams, 19, replaced Fabinho for the last hour and Klopp said he could be an option along with Nathaniel Phillips and Billy Koumetio, who have both been training with the senior squad.
"I can say a lot about his progress in the last few weeks or months," Klopp said of Williams. "He's been exceptional, he was already a very good kid when he arrived here.
"I realised long ago that life is a challenge. This is a challenge 100%... Problems are only there to find a solution for. I see the challenge in the situation and I see there's opportunity. We play football games every three days so this will stay a challenge... We will deal with the situation and make the best of it."
Liverpool skipper Jordan Henderson is fit, however, with Klopp saying he was withdrawn at halftime against Midtjylland in the Champions League to keep him fresh for the weekend.
"I can count the games he's played and we have to try to get through the period," Klopp added. "It is intense and hectic, like December but every month. So we have to make these decisions."
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