UEL and EPL are very different and both teams have competitive rivalries and even the final result cannot be predicted. Now we should not trust any team too much. In this UEL a weak team can be very strong and surprise us one day when they reach the final round. But I also will not ignore any team because in any case Man United still looks positive because of its achievements in the past. So some people still believe in the final result that Man United will get. While some people ignore the performance of Real Sociedad which can currently destroy Man United's defense.
Indeed, EPL and UEL are different but both of them are very competitive. In EPL, there are many strong teams, it is also the same as in UEL. However, in UEL, those teams rarely meet, so it is difficult to guess the power of each team. That's why the matches in UEL can be more difficult to predict. Sure, a smaller team can beat a stronger team. It is because most of us only determine the strength of the team based on their squad quality. If many popular players in the team, we will say it is a stronger team. However, in the match, sometimes there are other factors that can bring an influence. For example: the teamwork, the mentality of the team, and the tactics. These factors are not easy to determine, that's why our prediction is possible to be wrong although we choose a stronger team.
In the match Real Sociedad vs Man United, Man United surely has more popular players. But Real Sociedad sometimes can surprise us because the team from La Liga isn't easy to defeat. I think Real Sociedad can defeat Man United if they are not ready to deal with Real Sociedad tactics.