Linedelof, who was this month named his country's Player of the Year, had made six consecutive starts for United but misses Tuesday's Group H game against the Swiss after being injured in Saturday's 0-0 Premier League draw with Crystal Palace.
"(Victor will have a) scan tomorrow (Tuesday), 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," Mourinho was quoted as saying on the club website https://www.manutd.com/en/news/detail/team-news-as-jose-mourinho-rules-out-two-defenders-for-man-united-v-young-boys-on-27-november-2018.
"I would say if we have Victor available for the Christmas period when we need, if possible, every player to be available for that difficult period until the Newcastle match on 2 January, it will be good," he added on MUTV on Monday.
Lindelof has been joined on the sidelines by Argentine Marcos Rojo, who has "some little injury", leaving United with only three fit centre-backs for Tuesday's game.
“(Phil) Jones, (Chris) Smalling and (Eric) Bailly are the three that we have at this time,” said Mourinho, who added that full-back Matteo Darmian also has a slight injury problem.
The Italian played the whole match at home to Palace on Saturday as a replacement for Luke Shaw, who is eligible to face Young Boys after serving a one-match domestic suspension.
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