Derby aren't exactly a low-tier side.
"The story of the Premier League's worst team":
https://www.dailymail.co.uk/sport/football/article-5531005/DERBY.htmlbut it's probably best United being out of it so they can concentrate on getting back into form in the Premier League.
One of the biggest clubs in the world should have ambition, and it's expected that such team should win every single competition they're in, no matter how small they are. And given how bad things are within the club, I think the last thing they needed is getting KO'd by a small team in the league cup which is nothing but an embarrassment for the club.
We'll see how things play out from now on, but I don't believe this is the best that could've happened to United.