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.
I think even in theory it doesn't make any sense, because in case they can meet in any stage of the tournament, do they need a rival? Even considering it's MU, anything could happen in the moment. Moreover, despite MU's poor performances I think they still have a chance, plus they can't lose and need to hope for a miracle to qualify.