I can't say for sure if Bayern Munich will win against Manchester United. It is believed that Bayern Munich will not prove themselves weak by losing against a big team like Manchester United. They will definitely want to win the away match against United. However, since Bayern Munich have already qualified for the round of 16, this match is not important for them. Bayern Munich did not play to win against Copenhagen in the last match. Their performance was a bit slow.
This is not entirely correct; Bayern Munich played to win their last match against Copenhagen, but they were simply unlucky due to Copenhagen's defensive mindset. Copenhagen knew it would be difficult to beat Bayern Munich, so they immediately fought for a draw, which they obtained at the end of the game. When they beat Galatasaray in their final group stage match, the one point from this match will give them an advantage on the table and a better chance of qualifying for the knockout stage.
Bayern Munich's performance in past matches against Manchester United is very good and probably Bayern Munich team won 4 out of last 5 matches. So Bayern Munich team is considered as favorite in the upcoming match as well.
This is exactly what will happen; Bayern Munich will either win or draw the game. If either of these two outcomes occurs in this match, Manchester United will be eliminated from the Champions League competition, and they will not even be eligible for a Europa League spot if they do not win all three points in the match.