Entering F1 this year as a highly rated rookie, Belgian Vandoorne has in fact struggled alongside teammate Fernando Alonso so far in 2017.
Team boss Eric Boullier told El Pais newspaper: "In the lower categories, the car is the same for everyone so you adapt to it and then your style doesn't change.
"But in F1 there are always new parts and that changes the entire character of the car," said the Frenchman.
According to the newspaper, an unnamed team source was even more critical of the 25-year-old.
"He is good, but he's not a star. It is not possible to go to a circuit like Barcelona, where he has driven so many laps, and be so far behind his teammate," the source added.
"It is true that before his debut in 2007, Hamilton did a lot of testing and Vandoorne did not. But neither did Verstappen."
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