This is the exact point that I am emphasizing on, most people are underating United because they did poorly in EPL but they forget that EPL is tough and all those clubs have been playing with each other for a long time now. Lyon is a different club entirely from the EPL clubs which may give United an edge over Lyon because the club with the best strategy and performance will win the match.
I can understand what you are saying, but I don't think it's something that they actually "perform" well, it's just that they are in a very competitive English League so they are struggling.
We also have to look at the whole thing, how they play, what kind of mentality they show and so on. I'm not underestimating, but when I look at all the things I usually see to predict a team, then I see Manchester United are indeed in trouble, so that's what ultimately makes them not find their best game. and also we can see from the last few seasons how they have performed.