Given Manchester United's poor performance it's no surprise that bookmakers favour Real Sociedad, especially with the first leg being played in Spain. Real sociedad are a strong side and playing at home gives them an added advantage. If United wants to progress, the defending champions have to be more disciplined and consistent especially in away fixtures where Real Sociedad will look to capitalize.
Manchester United can try to play as best as possible and they of course don't give up hope of winning UEFA Europa League title. Firstly it is a big title and secondly it can help Manchester United be eligible for Champions League next season regardless of their low rank in Premier League at the season ending. They won't be too pessimistic about their chance and will keep fighting but I am as an observer, and not as their fan, really doubtful about their chance that is small in my opinion.
It would be very suprising if Manchester United wins Real Sociedad, I expect them to lose or draw, news have it that two key players, Urgate and Harry Maguire won't be available to play the match, it's a big blow for them since the youth players that would replaced them are not very experienced to play and major league and most who might start and coming from the substitute would be playing for the time.
It can happen as it is football, but I will be very surprised if Manchester United can do it. Real Sociedad is truly big test for Manchester United, that they are still quality enough to compete against strong opponents outside Premier League. Manchester United have good players but their confidence, mentality and combination on field in matches are super bad recent months. They won't likely wake up to win against Real Sociedad in my opinion, but perhaps they will try fighting and their fans have sort of hope.