Pierre Gasly says Red Bull went back on their word that he would see out the 2019 season in the team.
Gasly was vastly outperformed by Max Verstappen over the first part of the year, scoring just 63 points to the Dutchman's 181, with Verstappen winning in Austria and Germany while the Frenchman failed to secure a podium.
Alexander Albon has been drafted in for the remainder of the season, with Gasly demoted to Toro Rosso, as Red Bull ponder who should partner Verstappen in 2020. Albon, Gasly and Daniil Kvyat will all be under consideration.
Team principal Christian and motorsport advisor Dr Helmut Marko had said publicly that Gasly would not be replaced before the year's end, something he said was repeated in public, only for Marko to deliver the dreaded phonecall a few hours before his demotion was confirmed publicly.
Gasly told reporters: "I found out on the morning it was decided. Of course it was kind of a shock and [I was] disappointed because it's not what I had been told before or after Budapest.
"The team said I was going to drive until the end of the year, and that's what I believed."
A fourth-place finish in Bahrain last year vaulted Gasly into Red Bull's thoughts and he was selected ahead of Carlos Sainz when a replacement for Daniel Ricciardo was required and the Frenchman is determined to answer his doubters on the track.
"Whether I understand it or not is not going to change it," said Gasly.
"I didn't spend too much time thinking about it.
"The only thing I need to focus on is performing in these nine coming races with Toro Rosso at my best and this is what I am going to focus on.
"There are things I could have done better over that first half of the season.
"I'm partly responsible. I think all of us were responsible for the lack of performance and points we ended up having."
Asked if he had hope of returning to Red bull in 2020, Gasly said: "I am not thinking about it now, but that's what they say, and for sure there will be other opportunities."
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