Young Boys beat Manchester United 2-1 in their first group-stage meeting in September
Manchester United's Champions League tie against Swiss side Young Boys at Old Trafford on 8 December could be moved because of Covid-19 restrictions.
Concerns around the new Omicron variant mean anyone entering Switzerland from the UK has to isolate for 10 days.
Governing body Uefa is seeking clarification over whether that would apply to the Young Boys party.
If so, then Uefa rules state a neutral venue should be found, though the game could be moved to a different date.
United have already qualified as Group F winners but Young Boys could still secure a place in the Europa League if they emulate their victory over the Premier League side in September.
If the game was moved, it would cost United millions of pounds in lost gate receipts.
Switzerland is among a number of European countries to reimpose coronavirus restrictions amid a surge in cases across the continent, with sporting events also impacted.
West Ham's Europa League tie against Rapid Vienna in Austria last week was played behind closed doors as part of a new national lockdown, while all matches in the Netherlands are being played without fans after a partial lockdown was introduced earlier in November.
Meanwhile, Bayern Munich's game at home to Barcelona in the Champions League on 8 December could be played behind closed doors.
Markus Soder, the Minister-President of Bavaria, which includes Munich, has said he plans to prevent fans from attending sports events in the German state because of a rise in coronavirus infections – although he did not give a timetable for bringing in such measures.
"Football matches should be without spectators again. Bavaria will do that," Soder said on Twitter.
"Football has a great role model function. We must now reduce contacts everywhere."
Bayern have qualified for the knockout stages as Group E winners, with Barcelona and Benfica battling it out for the runners-up spot that will also secure a last-16 place. Benfica play Dynamo Kyiv in their final game in the same group.
- Our coverage of Manchester United is bigger and better than ever before – here's everything you need to know to make sure you never miss a moment
- Everything United – go straight to all the best content