That can't be a reference, Bilbao this season has performed quite impressively so it's not easy for Manchester United in this semifinal round even though many favor Manchester United but I don't think because of that the percentage will increase.
The first leg tonight Manchester United will play away which of course makes it even more difficult for them but if Manchester United plays safe and takes advantage of the second round later when playing at home maybe they can advance to the final, it's about strategy and it's worth looking forward to.
The strategy is to win and I'm sure both team coaches will give their best for this semifinal match because this is about not being so stressed when undergoing the second leg. In addition, Bilbao also really wants to be in the Europa League final this season so the team will definitely do everything they can to win early at home, while Manchester United is a team that will not allow its opponents to win early in this first leg because the second leg takes place at Old Trafford.