If you took Ibra out of Man Utd, I'm sure they'd struggle too.
It's natural to not be as good when your best player isn't playing.
I think the feeling is more that slipping down a division his loss shouldn't be felt as acutely as it is.
There's also the problem that all our other CMs (with Diame away, if he's even classed as a CM) are defensive midfielders to varying degrees. We wouldn't have looked as bad without Shelvey if we had anything resembling a replacement.