Not long ago, Tottenham and United were the tournament favourites, but apparently their "successes" in the Premier League forced the bookmakers to make their odds more reasonable. I don't know if Sociedad and AZ can knock them out, but I believe in Sociedad more. United had a grueling game with Arsenal, so maybe they won't have time to refocus on the Europa League. Besides, the overall result against Arsenal is positive, so it is difficult to expect United to show two positive results in a row 
As Manchester United showed a positive result in holding Arsenal in a drawing match in the Premier League, they can still display the same positivity when they face Real Sociedad, to at least win against them since they are the host team and the Europa League is the only league they have hope of lifting the trophy this season. If they fail to lift it, they have actually lost a lot this season, not making it to the top of the Premier League table, to contest for either Europa or Conference League next season.