As much as I dislike pretty much everything about Ronaldo's second stint, it really only scratches the surface.
Ole didn't sign a single central midfielder while in charge. And he had 3 summers to do so! Both Van Gaal and Mourinho at least attempted to solve the obvious midfield problem. They signed 3 central midfielders each I think. Now I don't know if this is entirely Ole's fault, but when you're stuck with McTominay and Fred as your most frequent midfield pair and don't sign any central midfielders then you are just looking for trouble. It worked for a while because our defense wasn't so bad and our attackers did really well. Once they weren't, you might as well have replaced our midfield with two cones. We were wide open every time we lost possession.
I also blame our English core to be honest. Maguire, Shaw and Rashford were absolute donkeys after the Euros. They were probably tired after not getting proper rest, but that does not excuse their sloppiness.