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.
United's performance will be difficult to predict in the upcoming match against Sociedad. Bookmakers favoring Sociedad certainly make sense. But Sociedad is also not a really strong team. I might see both teams have the same strength. So it depends on which team will take advantage of more opportunities. If Sociedad players can take advantage of United's defensive weaknesses, then they will benefit. But if on the contrary, United's attacking players play better than usual, Sociedad might be in trouble.