It's quite reasonable that Real Sociedad is more favored than Manchester United because Real Sociedad will have the advantage of playing at home, and as we know, Man United has inconsistent performance. But still, that doesn't rule out the fact that Man United could still make a surprise, because even though they are in ruins in the league, but what is happening in the Europa League is the opposite, where Man United can perform well and they get to the last 16 without having to go through the play-offs.
I see Manchester United's form as an anomaly. They have been very poor in the EPL but so far they have been one of the better teams in the UEL. United finished third in the standings and they are still unbeaten. Real Sociedad are the favourites in this match and in fact Sociedad have been better than United in the last few matches. But as you said United could spring a surprise in the match. But even so I still pick and support Sociedad to get the three points. They are playing at home so that will give them the motivation to win the match.