Real Sociedad is not okay this period and even if we assume that Manchester United is sick then I will be surprised to see Real Sociedad outperform them in tomorrow's game. Their last match was against Sevilla and they lost it, they were humiliated by Barcelona in the last days. Manchester United got a draw in their last match against Arsenal which is very good in the my opinion.
I will go with the team that fails its supporters the whole time, Manchester United!
Real Sociedad lost against Sevilla because they're playing with 3 main lineup and 8 second lineup, hence they can't even beat weak team like Sevilla. They humiliated by Barcelona because one of their players got red card.
However, the first leg Real Sociedad met Manchester United in Reale Arena, it was tie in terms of performance.
So, if they failed to dominating in their home, the second leg, it's obviously Manchester United will dominate on the pitch. Hence it's not surprising they're the favorite in this match.