Ole Gunnar Solskjaer says it is "touch and go" whether Marcus Rashford will be fit enough to return before the end of the season and warned he may miss Euro 2020.
Rashford suffered a double stress fracture in his back after coming on as a substitute in United's 1-0 win over Wolves in their FA Cup replay on January 15.
It was initially hoped he would be out for around six weeks but he now faces an extended spell on the sidelines, with Solskjaer admitting it is unclear exactly when Rashford will be fit enough to return.
"I would hope he's playing this season. It will just be touch and go towards the end," he said ahead of Manchester United's Europa League game with Club Brugge on Thursday.
"Hopefully we can get through this tournament and prolong the season. It takes time, I'm not a doctor. I was hoping he would recover quicker than what it looks like he's going to be out for - another few months definitely.
"If he's not fit enough he won't go [to the Euros]."
When pushed as to why the time frame for how long Rashford may out for had been extended, Solskjaer added: "He had a scan and it was a more serious fracture than we hoped and expected. He felt fine a few days before that.
"I didn't know it takes that time to recover. When it heals he will be stronger for it so it's important we don't rush it and we don't do that."
No return date in sight for Pogba
Paul Pogba is another high-profile absentee, with his recovery from an ankle injury continuing slowly, with questions remaining as to what sort of future he has with the club.
Pogba's agent Mino Raiola told Sky Sports News he will "reach out" to Solskjaer and insisted there is no bad blood between the pair following comments made about France international Pogba by the agent.
Solskjaer, however, focused his attention on Pogba's injury in his pre-match news conference, insisting he is not ready to return to training.
"He's not been part of team training yet, so it'll depend on how he feels or when he feels ready to do that," Solskjaer said. "He's not anywhere near that yet."
Ole hails 'top boy' Haaland
With Rashford out, Solskjaer's striker options are limited, although January Deadline Day signing Odion Ighalo has travelled with the squad to Belgium.
United had been linked with a move for teenage superstar Erling Haaland, who subsequently joined Borussia Dortmund, scoring two goals for the German side as they beat Paris Saint-Germain 2-1 in the Champions League on Tuesday night.
Despite missing out on the former Red Bull Salzburg forward, Solskjaer said he is pleased with Haaland's progress, especially as he is a Norway native like himself.
"I normally never comment but since I have had Erling I'm delighted for him," added Solskjaer. "He's a top boy, I want the best for him and he's started fantastically for Dortmund. For Norway it's brilliant we have a striker who can score again."
Sky Sports News reporter James Cooper in Brugge…
It is fair to say a lot of the feel-good factor following Manchester United's win at Chelsea almost evaporated in an instant when Ole Gunnar Solskjaer admitted Marcus Rashford will be out for "another few months" on Wednesday.
What Solskjaer's reveal, made at his news conference ahead of Manchester United's Europa League tie at Club Brugge, means is that there will be much conjecture about when and if the striker will be fit before the end of the season and whether he has a realistic chance of making the European Championships this summer.
It also underlines just how important the Deadline Day signing of Odion Ighalo was and how he is likely to be playing more football than even he may have hoped for, especially considering Mason Greenwood has remained in Manchester instead of travelling with the squad to Belgium.
Solskjaer says Ighalo has already settled into life at United and he looks likely to play a larger role in Brugge on Thursday night than he did at Stamford Bridge on Monday.
Paul Pogba was another topic of debate as he has increasingly become of late - expect that to intensify as the business end of the season approaches. The Frenchman needs to play for a variety of reasons but Solskjaer emphasised once again that Pogba is not training with the group at the moment and it is down to him to decide when he is ready. He then added that the player "is nowhere near that stage".
What team the United boss plumps for in Flanders is another interesting question. He has given youth a chance throughout the Europa League but there is an argument that Club Brugge, who have come into the competition from the Champions League, are the strongest opponents Solskjaer has faced so far in Europe. They have momentum and consistency too, top of their league with just a single domestic defeat all season and a coach who says his players are "willing to lay down their lives" to beat Manchester United.
The other consideration for Solskjaer will be which fixture of a three-game week is seen as the most important for United, who should have enough class to overcome Club Brugge and would like to continue their post-winter break push on Sunday at home to Watford.
Either way, this season has shown us on plenty of occasions that it is not easy to predict what kind of Manchester United team will appear for some games, both in terms of their approach and their personnel. A positive step forward in Brugge would hint at something more substantial and, after a short winter breather, the challenges are once again coming thick and fast for Solskjaer.