Nothing is impossible, just look like in the UCL, Dortmund were very lucky to reach the final and it was all because they had opponents they could beat even though Dortmund themselves were not strong team.
Something similar could happen to Manchester Uniter if they are lucky and only get opponents they feel they can beat, but opportunities like this are not easy to get, we see how things develop next season.
Manchester United played in the UCL last season and look how badly they performed. This season they will appear in the European league and I hope they can be a little better than the previous season. I think for now United is worthy to play in UEL. At least they got through the group phase and played in the quarterfinals or even the semifinals.
Dortmund are playing very well this season and I think they deserve to play in the final. Dortmund beat several strong teams such as AC Milan, PSV, Atletico Madrid and PSG to reach the final. Maybe they are not as strong as Munich or even City but Dortmund are playing extraordinary and could give a surprise tonight.
There is a difference when they play in the Bundesliga and when they play in the Champions League. Their game in the Champions League was more promising, so they managed to advance to the final match after beating Paris Saint Germain in the semi-final match. It was something that was not predicted beforehand, but they managed to do well.
Manchester United in the Champions League this season is very bad, they are the team at the bottom with only collecting 4 points. Even though previously they were favorites to qualify for the round of 16, they couldn't do it. They could only win 1 time and draw 1 time, and 4 other matches ended in defeat. Obviously it's a bad result to be in a group that tends to be easy in my opinion.