it is going to be a match to gain confidence or maybe lose it, depends on the result, but it's obvious that Man utd are going to win, and probably Real Sociedad too, so it very likely that Sociedad will be first and Man United will be 2nd by the end of the group stage phase.
In group E of the Europa League only two teams have been really consistent to get a win and that is also due to their good performance, because I see that Omonia and FC Sheriff are not so strong in group E of the Europa League which is very possible not to qualify for the Europa League. round of 16 for both (Omonia and FC Sheriff).
And overall, Manchester United only need to win two more games in the group stage to ensure they qualify for the next round. So Manchester United also won't be bad if it has to be defeated by Real Sociedad in the group stage if Manchester United can beat Omonia and FC Sheriff.