Mentality might be a factor that will help, that is if their mentality is very good. But looking at their performance this season or previous seasons, I can conclude that they are not a club with a mentality. They are different from the Manchester United we used to know, in the past they were a club with a very good mentality, they once made a miracle in the Champions League final against Bayern Munich where they were able to turn things around in a short time. But then again that was then and now they are not the Manchester United I used to know.
Manchester United hasn't lost a game in the Europa, they drew 3 games and won 5 games, it's a good stat although only in the Europa since they have been incompetent in the Premier league, and over all performance isn't something to rely. Manchester United was fun to watch, the epic performance of the team was something everyone loved to watch, including rivals now they are just some random team that plays their games and loses points. They might succeed in the Europa, the teams in the league are not better rivals that we can ultimately cast on to win the title, only Manchester United and Tottenham who look like the major clubs to win at their best.
Their statistics in the league round cannot be a factor that guarantees that they will be able to do the same, especially in the league round they were not so impressive. Yes, they did not feel defeated, but overall they were not impressive and showed that they were ready to win the title. While in this knockout round they will meet the best club that managed to qualify from all the participants in the first phase. Also some clubs I think are better than them overall.