I think you'll agree Leverkusen were absolutely awful at Old Trafford, they just weren't up for it. That was by far their worst performance this season, let's not forget they are third in the Bundesliga and having by far a better season than Man Utd. A Man Utd on top form would have some trouble negotiating them away from home, this team is going to be overwhelming underdogs there.
Leverkusen should've beaten you at Old Trafford as well, that's how they've been playing this season in comparison to you guys, that was just an awful, awful performance though so they got crushed.
People like to underestimate Bundesliga for some reason it seems. The top teams are a lot stronger than the best English teams, that's obvious, and teams like Schalke and Leverkusen are quality sides.