To be honest, a young midfielder in his first year for the biggest club for England isn't going to set the world alight if his chances are sporadic.
Paul Scholes had spoken on it too, even though he's a bizarre pundit at times I think he knows his midfielders and what helps them - and he has also said VDB needs games in succession to be able to make an impact and find that chemistry with his peers. Playing one game, then not the next 7, then playing 15 minutes at the end of the game, the playing cup matches with the second string etc, that's not going to help him.
Of course there is an element of him being wasteful too and not taking the chances proactively whilst he's on the pitch. For example he shone against Southampton and was one of the best players on the pitch by far, then he was anonymous vs West Ham. So yes there is an element of him needing to do better but I think if he was given more consecutive chances we'd see him more consistent.
Midfielders often need to play themselves into the team to get on that consistent run they are capable of. We have seen many midfielders take a long time to settle into the team and get consistency - be it Fred after his terrible start for us, or better players like Fabinho at the start for Liverpool, or Rodri for City.