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.
How would anyone even be suprised by that, don't they watch Manchester United's matches and how boring their performance is? The bookmakers are aware that Manchester United got series of injured first team players and also performing very poorly, moreover they've not even very consistent recently and would rely on the help of junior team players to make up the substitute and starting 11 player for the match tomorrow.
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.