I think this season Sevilla was very lucky to defeat Manchester United . Manchester United have done well in their performance this season, Sevilla was just lucky that the time they met with Manchester united , Manchester United was in their worst state , some players were on injuries and other mistakes that led to the defeat of Manchester United.
The main reason people think Sevilla is a strong team is they just see the results they get and they don't pay attention to the situation of the games they have, for example, we know against Manchester United they had no chance and the reason why they won the game was just because of the personal mistakes in Manchester United defenders, otherwise, Sevilla could easily lose that games against Manchester United.
I think that series was purely based on luck for them, that is a very fun situation to be in and definitely something I did not expected. United fans must be incredibly angry at it but between mistakes by maguire and de gea on second game, and own goals at the first game, it wasn't really Sevilla that won the game, they did nearly nothing, they just took advantage of the situation, they may have failed to do that too, but they didn't, that's their only achievement.
United was the who lost that series and definitely looks to be a very tough deal. Sevilla is not a great team just because they won against United, that was lucky, I think they are doing alright, and they will keep on doing alright, but they may fail to win the cup, they won't get lucky like that again.