Jose Mourinho expects Victor Lindelof to be out for "quite a while" with an injury that compounds Manchester United's defensive problems heading into a crunch period and important Champions League clash with Young Boys.
Recently crowned Sweden's player of the year, the ever-improving 24-year-old has established himself as one of the first names on the team sheet, starting the club's last seven matches in all competitions.
But Lindelof's resurgence after a slow start to life at United has been derailed by a muscle issue, meaning Mourinho's remaining centre-back options are Chris Smalling, Eric Bailly and Phil Jones.
Marcos Rojo – yet to play a minute this term – is also absent for the United manager, who has made his frustration clear at the club's failure to reinforce the defence over the summer.
"He will be out for quite a while," Mourinho said of Lindelof ahead of Tuesday's Group H clash with Swiss champions Young Boys. "We have Bailly and Jones.
"Marcos Rojo has some little injury and Victor an injury – a proper injury – that will keep him out for a while, so Jones, Smalling and Bailly are the three we have at the time.
"(Lindelof) does the scan tomorrow but, by the experience of the medical department and the feelings of the player, he has a muscular problem and a muscular problem takes time.
"I would say if we have him for the Christmas period – where we need, if possible, every player to be available for that difficult period until the Newcastle match on January 2 – if we can have Victor available for that period, it will be good."
Lindelof was one of few players to emerge from Saturday's 0-0 draw at home to struggling Crystal Palace – just United's second Premier League clean sheet of the season – with any credit as their Old Trafford troubles continue.
Mourinho's men have won just three of nine matches on home soil, but the manager knocked back suggestions his players find it easier to play away.
The United boss also laughed off a suggestion that playing at Old Trafford brought added pressure ahead of a match that can see them progress to the Champions League last 16 should they beat Young Boys and Valencia fail to win at Juventus.
"If you feel pressure, stay at home – and when I say home, I don't say stadium home, I say home home and watch on TV," Mourinho said.
"If you feel pressure to play matches at home where the people come to support... come on.
"I never felt pressure to play at home."
Mourinho expects a response from his side on Tuesday, having questioned some of his players' "heart", desire and intensity after the tepid draw with Roy Hodgson's struggling Palace.
The United manager stopped short of using such cutting soundbites on the eve of the Young Boys encounter but shed a light on the issues he faces.
"It's frustrating because what I want is the team to start strong, is the team to go strong on the opponent, not to be waiting, not to be reactive but proactive," Mourinho said, having also said his side are "not very good on second balls".
"That's exactly what I want and probably you or the fans, they think 'oh, Jose told them to start slow', 'Jose told them to play nice and easy and wait to see what happens' or 'to be losing for then to react'.
"It is exactly the opposite thing. I want the team to start strong, I want the team to have a go immediately and it is a bit frustrating."