I wouldn't underestimate Athletic Bilbao and think they are even a better team than Manchester United. They are actually 4th in the league and in a CL spot while ManU is in the bottom third of the table and having one of the worst seasons in the club's history. If ManU had played Bilbao a few years ago they would have been the favorites but today I think Bilbao is the favorite.
Yes, I agree with you, that in this match Bilbao is more worthy of being the favorite, because Bilbao has a better achievement in its domestic league. While Manchester United, in its domestic league played very badly and had to fight hard to avoid being relegated. That is why I also agree that currently Manchester United cannot be favored like before, because their performance has also been real proof that they are no longer the favorite team at this time. Because in the English league Manchester United has not been able to become champions for a long time. So with that I think it is only natural that currently Manchester United's caste has decreased slightly and is not as famous as before. Because after the Sir Alex era ended, until now Manchester United has not been able to restore its glory like in those days. So in the match against Bilbao, I will bet on Bilbao only, because in my opinion choosing Bilbao in this match would be much more appropriate.