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.
Manchester United have been able to hooked up with streak winning in the Europa League but I'm afraid we can't say the same for them in the English Premier League. The Red Devils have been scuffing this season with unknown stats to bounce back to astounding form. Ruben Amorim working hard enough to ensure he's making some piece for the growing condition of the club, he still need more time to settle and present himself worthy for the incoming goals he stand to accomplish for Manchester United.
The Europa League have been a major target for the club to lay their hands on since they're not having a good running in the EPL. Atleast the Red Devils can balance the scale if they're able to pull a win for themselves, always enabling themselves to come infront of big winnings. There's also something to know about Manchester United, they can zipped up means and be able to inculcate wide span of winnings.