It is difficult to make predictions about this match. I would say that the situation of both teams is very similar. Both teams are weak in their own league, but they are playing quite well in the Europa League. Neither Real Sociedad nor Manchester United are in their best form. So it is not possible to say for sure which team will win this match.
Both teams are not in similar situations. Real Sociedad is clearly a better team than United currently. Their last two losses shouldn't fool you, they played against the 2 best teams in Spain (Madrid and Barcelona) so I wasn't surprised they lost both those games.
Socided play much better football than United and they have a system and a style of play, but Manchester United doesn't. I don't even recognise them when I watch them.
I'm not saying Real Sociedad will definitely win Man United, but if it comes down to it, I'm picking Sociedad to go through after both legs. The problem Real Sociedad might face is that they have a couple of players injured and I don't know if they're going to be available for the match on Thursday. If the whole team will be available then I'll bet on a Sociedad win.