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 actually doubt Bayern would give that up because it would be tarnishing their unbeaten group stage record.
It should be realized that Bayern is the only club that has never tasted defeat in the group phase for the last 6 seasons because they have tasted defeat in the group phase in the 2017/2018 season and even then only one time they lost so with achievements like this it is clear they will not return to defeat just because they want Manchester United to have a great opportunity to qualify because even if Manchester United win but one of Copenhagen / Galasaray wins Manchester United will also not be able to qualify even though there is a guarantee of entering the Europa League.
I don't think Bayern will have their unbeaten group stage record ruined this season.