Mac Allister joined Liverpool from Brighton during the summer
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.
"The other boy stepped on his knee and the stud went through pretty much the muscle onto the bone.
"Now, we have to wait until the bone is healed and until Macca can deal with the pain, because it is pretty painful.
"Macca is actually a super-tough guy so we have probably to make sure just from an infection point of view nothing happens. That's really important.
"I never had this injury for one of my players before but now we have it.
"I hope it will not take that long, I would like to say it's day by day - but that would look like he could maybe play the next game but he will not be ready for that as well.
"I hope the next three or four days he makes big steps and then we will see."
Liverpool have already secured top place Group E and qualified for the knockout stages going into the game against Belgian side Union Saint-Gilloise.
Klopp added: "The line-up in my mind and the line-up we used in training would mean we have a good mix between experience and youth. We want to go for it, that's it.
"We have a really good group of young boys [in the Under-23s] and some of then we shall see tomorrow, that's clear."
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Source: bbc.com