Liverpool manager Jurgen Klopp said he has never had the type of injury that has sidelined Argentina midfielder Alexis Mac Allister.
Mac Allister suffered a cut knee in a win at Sheffield United on 6 December.
He missed the victory at Crystal Palace on Saturday and has not travelled with the Reds for a Europa League group game at Union Saint-Gilloise on Thursday.
Mohamed Salah, Virgil van Dijk, Trent Alexander-Arnold and Alisson Becker have been rested for the tie.
"It's a bit more tricky than we thought at the first moment," said Klopp on Mac Allister, who is also expected to miss Sunday's Premier League game against Manchester United.