Red Bull advisor Helmut Marko believes Daniil Kvyat had more natural talent than Toro Rosso duo Brendon Hartley and Pierre Gasly, but “something happened to him mentally”.
Kvyat started his Formula 1 career with Toro Rosso in 2014 and was promoted to Red Bull for the 2015 season, but he was demoted after four races in the 2016 campaign with the team opting to give Max Verstappen a chance.
He returned to Toro Rosso for 2017, but was dropped again at the backend of the campaign and released by the Red Bull driver programme.
The team have decided to go with the Hartley-Gasly partnership for next year.
When asked by Autosport if he thought Kvyat had more talent than the current STR drivers, he replied: “Yes, I completely agree. Unfortunately he just showed it in his first year with Toro Rosso and in his first year with Red Bull Racing.
“After that his performance drastically went down. He had brake and tyre issues, that [Daniel] Ricciardo didn’t have.
“In short, something happened to him mentally. He lost his speed and his ease. We don’t know [what it was].
“We tried many things, but his speed just wouldn’t come back. Unfortunately we have no idea what happened.
“There were also too many accidents at the start of a race, and he didn’t react too well to it either.
“He retreated into his shell and didn’t want anybody to tell him what to do.”