Manchester United has faced Athletic Club in 3 matches before. Manchester United could not win in any of the 3 matches. However, these matches were many years ago. Now, maybe the scenario has changed. However, looking at Manchester United's overall performance, I do not think United will be able to defeat Athletic Club in the first leg match. Athletic Club's team is quite strong. Even this team can be put ahead of United.
By looking at MU current situation, they are -8 goals in the EPL, meaning that apart from the goalkeeper who concedes easily, the attackers are also less productive. I know, that H2H doesn't guarantee the current result, but by combining the situation and H2H, one short prediction can be made, it looks like MU will find it difficult to win this match. Things will definitely not be easy for MU considering they are in 14th position in the EPL, of course there will be mental pressure experienced by all the players. This could be proof that they only have difficulties in the EPL but not in the UEL, or it will add to MU decline this season.