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.
Manchester United played well in the EL, compared to the EPL they have much better results here, but with Real Sociedad everything can be different, they are a home team, they are very strong on their field and Manchester United in this condition is not a very dangerous opponent. I think that Real Sociedad will not lose this match, if Amorim manages to play this match in a draw then it will be a good result.