There are a few reasons:
Fergie was a genius and impossible to follow. After he left a bit of the aura of United vanished with him. This was made worse by appointing Moyes which eventually made us into a laughing stock. Getting Moyes to follow Ferguson and having to endure his weak personality...
I thought Fellaini was one of our better players. He kept his passing simple and effective, usually forward. I thought Herrera was worse, but I was watching it on a shitty stream, so I might have missed something.