Eric Boullier said that Vandoorne hasn't performed the way he expected.
When Sky Sports asked how pleased are people at McLaren with Vandoorne's performance so far, Boullier said: "Not as good as I was expecting. He's got a pretty high track record in the junior categories. He won everything that he went through like Lewis [Hamilton]. He's very talented.
"We didn't give him 100 percent to do well because he had a lot of issues on his car which was quite regular. But we are now addressing this to help him drive the cars the way we want to be fast. I am confident he will achieve this. All the young drivers starting from Verstappen struggle in the first six months."