I agree with you. Although the situation is getting difficult for Man United but the chance isn't over yet. Winning against Bayern Munich isn't impossible if Bayern Munich won't play with their main players and with serious efforts. Bayern Munich possibly use their second tier players because they already qualified for the next round. I think Bayern Munich will prefer to let their main players to take a rest. Meanwhile for the match Copenhagen vs Galatasaray, it is difficult to predict the result of the match. But it is surely not impossible to end with a draw. If both teams play all out, a draw can be the reasonable result.
Bayern Munich will likely form their second tier squad. Even so, Manchester United's chances of winning are slim, even with Bayern Munich's second-tier squad. Their defense is quite weak. On the other hand, Bayern Munich plays attacking strategy. Manchester United's defense is unlikely to improve in front of Bayern Munich's attack. I don't think United will win this match. But since the match is not important for Bayern Munich. So the match is likely to be a draw. Bayern Munich drew goalless against Copenhagen.
That focus must be done by Manchester United at this time, they do not need to see the results of the Galatasaray vs Copenhagen match, what they have to do is how they win this match. Although their fate is in the hands of the results of the 2 teams, but for me it will depend on their luck, whether they will have that much luck to see Galatasaray vs Copenhagen draw or not.
I don't know for sure whether Bayern Munich want to tackle Manchster United or not, because if you look at their previous results, they are not too ambitious to win, but I don't know in this match.
Manchester United still have a chance to qualify for the next round. But for this they must win. They need to get 3 points against Bayern Munich. At the same time, Manchester United can qualify for the round of 16 only if the match between Galatasaray and Copenhagen is a draw.
Manchester United have not won a single match in their last 5 meetings against Bayern Munich. They have lost all 4 matches and United managed to draw only one match. That is why the probability of Manchester United winning against Bayern Munich is very low, even though the match is not important for Munich. Even though Munich's second-tier squad plays in this match, I consider the Munich team to be the favourites.