Marcus Rashford is not certain to play again this season and at serious risk of missing Euro 2020, in a potentially huge setback for Manchester United and England.
Ole Gunnar Solskjaer said it was "touch and go" if Rashford would return for United before the end of the campaign, after admitting he underestimated how long it would take for the striker to recover from a double stress fracture in his back.
The news is a crushing blow for United as they bid to avoid a second successive season out of the Champions League, while England could be without one of their key players for the European Championship.
Solskjaer insisted Rashford will not be rushed back given the seriousness of the problem and would not feature at the Euros if he has not played for United before then.
"I would hope [Rashford] is playing this season, but it'll just be touch and go towards the end," Solskjaer said before the first leg of United's Europa League round-of-32 tie against Club Brugge in Belgium tonight.
"Hopefully we can get through this tournament and prolong the season. It takes time. I'm not a doctor but obviously I was hoping he would recover quicker than it actually looks like he's going to be out so another few months definitely."
As well as Rashford, it remains to be seen if midfielder Paul Pogba plays for United again this season. Solskjaer said that Pogba, who is eyeing a move to Real Madrid or Juventus this summer, was "not anywhere near" a return to first-team training after undergoing ankle surgery last month.
"He's [Pogba] not been part of team training yet, so it will depend on how he feels when he's ready to do that. He's not anywhere near that yet," said Solskjaer, who could hand loan signing Odion Ighalo his first start.
Arsenal: Mesut Ozil will not travel for Arsenal's round of 32 first-leg clash with Olympiacos, with his wife is close to giving birth.
Matteo Guendouzi returns to the squad having missed Sunday's 4-0 league win over Newcastle amid rumours of a fall-out with head coach Mikel Arteta, who takes charge of his first European game. Lucas Torreira is a doubt but Pablo Mari could be in line for his debut.
Celtic: Neil Lennon will have to do without Leigh Griffiths after the striker picked up an injury ahead of tonight's clash with FC Copenhagen.
Celtic are in flying form, with nine wins in a row while the Danish side, despite numerous injuries, have decided to sell Cyprus international Pieros Sotiriou, who has scored 13 goals this season, to Astana before the first-leg clash at the Parken Stadium.
Club Brugge v Man Utd, Virgin Media 2/BT Sport 2, 5.55; Copenhagen v Celtic, VM/BT Sport 3, 5:55; Wolves v Espanyol, VM/BT Sport 8.0; Olympiacos v Arsenal, VM/ BT Sport 2, 8.0.