Liverpool midfielder Arthur Melo is set to be sidelined for three to four months as he undergoes an operation this weekend.
The Brazil international has tasted just 13 minutes of first-team action since joining the Reds on a season-long loan from Juventus on transfer deadline day, with all of those coming in the 4-1 Champions League defeat at Napoli last month.
He was then an unused substitute in the matches against Ajax and Brighton, but wasn't on the bench for Tuesday's Champions League win over Rangers.
Jurgen Klopp confirmed on Friday that Arthur had sustained an injury in training earlier in the week, but refused to elaborate on how long the 26-year-old would be out for.
However it has now been determined by Liverpool's medical staff that Arthur needs an operation on a pulled quad muscle, and he won't play again until January at earliest.
The news will come as a huge blow to Klopp and Arthur, who was desperate to shake off his injury-prone image after a couple of stop-start seasons with Juventus, and prior to that at Barcelona.
He had brought his own fitness and conditioning coaches with him to Liverpool in a bid to impress enough to earn a permanent deal, and he played in matches against Leicester City's under-21s and Rochdale in the Papa John's Trophy in a bid to get up to speed.
Liverpool paid around £4m to take Arthur on loan for the season, and are believed to retain an option to buy him for around £33m in the summer.
The 22 times capped Brazil international arrived at a time that the Reds were suffering from several injuries in midfield, with Klopp previously insisting that they didn't need to sign another midfielder.
Jordan Henderson, Thiago Alcantara, Curtis Jones, Naby Keita and Alex Oxlade-Chamberlain have all suffered issues this season, with Keita and Oxlade-Chamberlain both still sidelined ahead of Sunday's trip to Arsenal.
Discussing his side's current injuries on Friday, Klopp said: "Curtis Jones trained yesterday and looked good, just the first session.
"Naby and Ox still a bit away [from returning]. Arthur is injured in last session before Rangers. Normal situation, a muscle. Andy Robertson is running, looks close. Curtis is closest [to returning]."
