Benjamin Sesko will be out of action "for a few weeks" after hurting his knee before the international break, manager Ruben Amorim said on Friday.
The forward, who joined United from RB Leipzig for £74 million in August, was injured during the 2-2 draw at Tottenham on 8 November and missed Slovenia's World Cup qualifiers.
Amorim was initially "more concerned" about the issue due to the fact it related to a knee, but the United boss is confident the 22-year-old has avoided a lengthy lay-off.
"He is going to stay out for a few weeks, I don't know how long, but it is not that serious," Amorim said at his pre-match press conference for Monday's visit of Everton.
"We have to be careful with him, he is going to recover, he is doing the recovery and is feeling better. So in a few weeks I expect to have Ben (Sesko)."
Amorim said he expected Sesko to return around the time that Cameroon's Bryan Mbeumo, Ivory Coast international Amad Diallo and Morocco's Noussair Mazraoui would leave for the Africa Cup of Nations, which begins in Morocco on December 21.
Discussions are taking place about when they will be released.
"We are doing our job to try to maintain the players a little bit longer but it's not just in our hands," said Amorim.
"We need to respect that, but we will try to do everything to protect the club, protect the player and protect the national team."
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The Portuguese was asked how he felt as he approached the anniversary of his first game as United boss – against Ipswich on November 24 last year.
"Right away, I knew that we will struggle in some things," he said. "But it was that feeling of, yes, it's the best league in the world.
"It's maybe the best club in the world. But at the same time, it's a big job and it's going to require a lot of work."
Amorim said United were showing progress after a tough 2024/25 season, when they finished 15th in the Premier League.
"We suffered a lot last year, so we have to take that into account and play every game like it's the last one or the first one," he said.
"But of course, if you ask me, I expect a team with more control, more dominance, playing better football and, the most important thing, more competitive in every way."
United will also be without Harry Maguire against Everton after the defender picked up an injury in the Spurs draw.
Midfielder Kobbie Mainoo could be available to face David Moyes's side and Lisandro Martinez is getting closer to a return, after being involved in the Argentina camp over the break.
United are seventh in the Premier League after an unbeaten run of five games, although they have drawn their past two, against Nottingham Forest and Tottenham.