It depends if Bayern decide to rest some of their stars as they are already group winners. Maybe they might want to give some players a rest. If that happens and Man Utd play the perfect game they could win.
It is very possible that Bayern Munich coach (Tuchel) sends some of their key players to the bench. Basically, the last match won't impact anything to their status, Bayern Munich will be the winner of group A whatever the result of the last match. I'm sure Tuchel will prefer to put substituted players for the lineup against Man United. However, it won't be an easy match although Bayern Munich will play with their substituted players.
Unfortunately for Man Utd it might not be enough, even if they were to win. If the other game has a winner then Man Utd are out. If Man Utd lose to Bayern they won’t even drop into the Europa League.
The match between Galatasaray and Copenhagen will be the key. Man United only has a chance to qualify if the match ends up with a draw. But if it doesn't end up with a draw, Man United has no chance to finish in 2nd place.