Toro Rosso says Gasly/Kvyat 2018 line-up not a given

Gasly made his grand prix debut with the team in Malaysia, standing in for Kvyat, and is expected to see out the season - even though doubts have emerged over his presence in the United States Grand Prix due to it clashing with the Super Formula finale.

Carlos Sainz will switch to Renault for the Austin race ahead of a full season with the French manufacturer in 2018, leaving Kvyat and Gasly as the obvious candidates for the two seats at Toro Rosso.

However, Tost has insisted nothing is set in stone for next year in regards to his team's line-up.

"So far, next year is open," Tost told Motorsport.com. "We will see, we have time. Red Bull does not need to hurry up with this, and the rest, we will see.

"Let's wait now for what's going on in Austin. There's nothing else to say."

Tost downplayed suggestions that the company may yet look outside its own pool of drivers for 2018.

"There are always options, but the question is whether you stick to a philosophy or not, and the philosophy of Toro Rosso is always run Red Bull drivers," he added.

"The drivers in the end are being decided by Red Bull."

Tost admitted Kvyat was obviously frustrated at being benched for two races.

"Of course every driver is disappointed when he's not driving races, but there were some reasons," he said.

Regarding Sainz's early move, Tost said: "It was decided he's going to Renault, there's nothing more to say. That was part of the deal.

"We gave him to Renault as compensation for an existing contract between Toro Rosso and Renault, to get out of this contract, and this was negotiated in this way, and therefore it was to be expected."