Liverpool midfielder Harvey Elliott has undergone surgery in London on Tuesday to repair a dislocated ankle sustained during Sunday's Premier League victory against Leeds United at Elland Road.
Liverpool said in a statement that the 18-year-old is expected to return to action later this season, although no timescale is being placed upon his comeback.
"We won't put pressure on him by setting a specific timeframe beyond being able to say with confidence we expect him to feature again later in the season following our rehab programme," said club doctor Jim Moxon.
Elliott was injured following a second-half challenge by Leeds defender Pascal Struijk, which resulted in a red card for the Dutch player. Leeds confirmed on Tuesday that they would appeal the sending off.
Struijk, who was sent off immediately after, apologised later on Instagram, saying what happened was something he "would never wish on anyone."
Elliott responded by saying it was a freak accident and was not worthy of a red card.
"I'll be back stronger 100%. Thanks for all the support," he said.
"I spoke to him after the game," Liverpool manager Jurgen Klopp told a news conference on Tuesday. "He was in the best possible place at that moment and he knows he will be out for a while, but he accepted it already.
Although the two clubs, who have won 13 Champions League titles between them, contested memorable finals in 2005 and 2007, they have never previously met at Anfield.
"In any kind of European battle, I think Liverpool v AC Milan is a must-watch," Klopp added. "Milan have not been in the Champions League for a few years, but they have been doing well for a year-and-a-half, they were brilliant last season and have started this season well, so it will be tough for both sides.
"There are a few nice games in this round of the Champions League and Liverpool v Milan is definitely one of them.
"It will be difficult. When we saw the draw, I don't think one of us said 'it's a great group.' But let's go for it.
"Atletico, Milan, Porto -- usually the top team in Portugal, Milan with their best side in years -- but that is what we get.
"This is the strongest group I have ever had at Liverpool. It will deliver a really strong team into the Europa League."
ESPN's Mark Ogden and Reuters contributed to this report.