Na, he's a very good defender, but I think fans have gone overboard with the praise. The whole inspitstional leader stuff is bollocks and has gone to his head a little, though, I agree with that.
As a defender, he makes many more mistakes than is acknowledged, especially when he has runners into the box. He loses his man frequently and is prone to making bad decisions when he doesn't have time to decide. Yesterday is an example; I didn't see a single person anywhere criticise him for the fact that he completely lost Sterling for City's goal. Mustafi would have been sluaghtered, and rightly so.
I think Liverpool won't be as good this season as last and van Dijk, along with a few others, will be thought of a little less highly a few months from now.