Toro Rosso team principal Franz Tost says Daniil Kvyat's departure from the team was due to both parties losing confidence in each other.
Tost confirmed on Friday that the Russian had been released by Red Bull's sister outfit, with Pierre Gasly and Brendon Hartley likely to form Toro Rosso's driver line-up for 2018.
Kvyat was out-performed by team-mate Carlos Sainz prior to the latter's recent switch to Renault.
"After Austin, he had five points," Tost told Motorsport.com. "If you compare it with Carlos, although he was not racing with us in Austin, he had 54 points.
"Both parties lost confidence in each other. In such a situation, it's better to say OK, we stop the co-operation and this gives Daniil the chance to look for another team and to look for his future.
"We felt the confidence was not there any more on the level you need if you are to work together successfully in the future."
Tost did not rule out the possibility of Kvyat one day racing for Toro Rosso again, adding: "In motorsport, especially Formula 1, you should never say never. Daniil has the talent, he's very fast and if he gets everything together, he can be successful."