The departure of technical boss James Key hurt Toro Rosso this year.

That is the admission of Pierre Gasly, the Toro Rosso driver who is switching to the senior Red Bull team for 2019.

It has been an odd season for Toro Rosso, as Honda steadily worked on its engine in anticipation for 2019, but development of the Faenza team's chassis actually stalled.

"We didn't really have any upgrades on the car since Melbourne," Gasly is quoted by Globo. "Or there was a very small one in Monaco and the one in Austria didn't work."

He thinks Key's impending switch to McLaren hurt Toro Rosso, because the highly rated engineer is already on so-called gardening leave.

"You could say that if a team loses its technical director, that will always have an impact on performance," said Gasly.

But the good news for Gasly is that Toro Rosso is basically being used as a 'test car' for Honda's rapid engine development for 2019.

"We know that if we introduce more engines we will have penalties, but certainly with the new specifications, we will have a better chance of fighting for better things in the last races," he said.