In the game Rock-Paper-Scissors, Rock is superior to Scissors, Scissors to Paper, Paper to Rock.
Whilst I am not going to try to apply the analogy to United and Bilbao I do think, like many on here, that we got found out against Athletic Club. Their high pressing game destroyed us, many saying they beat us more thoroughly than when we played Barcelona both times. And I agree.
If it was a matter of life and death, or Champions League or not, we would probably raise our game and give them a good show of it, so we could probably beat them about as much as they beat us.
No one is suggesting that Bilbao are going to win Leagues against us, assuming that we where in the same league, but they found us out. They destroyed us. So in a very basic "play each other to decide who is better" kind of way, they are better than us.
And those who are saying "we have a much better squad", so what? City have a better squad than us. Are they better?