Realistically, seeing how the united team has been performing since this season, Sociedad will definitely give Manchester United a very tough time in this game despite the fact that the red devils are playing host. And yes, just as you've said regarding their clash with Arsenal during their Premier League game, they may have really lost focus of this game having them only few days to get prepare, So, some of their players might be weak, whereas on the other hand, that game may also have booster their moral a bit to face Real Sociedad in this their encounter.
The situation of United and Sociedad is similar. We can see weak performance from both teams in their own league. However, both teams are trying to win the Europa League title. The first leg match was very competitive . Both teams tried their best to win.
The second leg match is Manchester United's home match. However, considering the home match, Manchester United team cannot be called strong. Because in the home match too, we have seen Manchester united team lose to weak team. So it is impossible to predict which team will win this match with certainty. Both teams have the possibility of winning. Betting on this match is risky.