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Anything is possible, but not in this case with Man United, and which angle of possibility are you talking about?
Bayern Munich in talk is inform and hasn't lost any of their games in the group stage match, even from record, Manchester United hardly win Bayern, Manchester United in quote is not inform both in their performance in the Premier League and in the championship league games, they are currently in the bottom of the table in the Championship league group stage game and are facing Bayern in their last game, whereas Galatasaray and Copenhagen are meeting, and both teams are ahead of United in points, so any of the team that won in their game, it over for United even though it magically happened that Man Utd won the German side Bayern.
On paper, Bayern Munich's performance is not in doubt, but considering that they are safe from qualifying and leading the group, I think they will not be shy about keeping some of their best players. Bayern Munich will be favorites to win the game if they play their best squad, but if they don't then Manchester United are the favorites.
I'm not sure why you are so pessimistic about Manchester United in their home game, but of course they have to work extra hard to win the game. Bayern Munich will still try to avoid defeat and maintain its winning record, but this is not an impossible mission for Manchester United.