Actually for Bayern from the 5th match they are also safe so they are not too eager to get 3 points in the previous match because after all they need to save energy for matches that are not too important.
You are right because with the points Bayern Munich have right now there focus is not much on there match against Manchester United because they will like to reserve most of there key players so that they will have enough energy to perform well on there next round, so that will make them use there younger players against Manchester United.
So however if Manchester United utilizes the opportunity and win Bayern Munich they could stand a bit chance of hopping for the second position if Galatasaray and Copenhagen plays draw.
Not entirely sure Bayern Munich will give Manchester United an easy victory, because even though Bayern Munich has qualified for the round of 16 but I am sure. Of course Bayern Munich will still play their key players, although maybe not completely but at least Bayern Munich will also try not to be defeated by Manchester Untied. Because yes, of course Bayern Munich also has prestige and thus, then Bayern Munich will not play elephant football to let Manchester United win easily, I am sure that will not happen.
Even though Bayern Munich won't give Manchester United an easy win, that won't make them fixture their key players in a group stage match they have qualified already. No football coach would ever do that to play his key players in a match they have fewer worries about in qualifying. They will prefer to fixture their second-team players with some first-team players together.
I suspect that's what will happen from Bayern Munich's side, while Manchester United will play with their full squad to make sure they win the match by all means