Pierre Gasly has said he "cannot understand" why he has missed out on a Toro Rosso seat for 2017.
The Frenchman was overlooked in favour of Daniil Kvyat, who has had an up and down season.
Gasly currently sits behind Antonio Giovinazzi in the GP2 standings, and is seen as the cream of Red Bull's young driver programme, but he too has had an inconsistent 2016 campaign.
Red Bull team boss Christian Horner recently revealed that the energy drink company was now looking at even younger prospects for the future.
"We are looking at the 13 and 14 year olds," he said, "because otherwise there's not much there."
But Gasly, 20, told French television Canal Plus that he "cannot understand" Red Bull's decision not to promote him.
"I talked a lot with Helmut (Marko) on Friday," Gasly said in Austin.
"He made it clear that things were moving in the wrong direction, but it was a complete surprise to me.
"On Saturday morning he told me about Kvyat, but honestly I cannot understand this decision. Especially as I had a long talk with Helmut but he did not explain to me the reason.
"Now we must continue to look ahead and explore other options, but I will prove to them that they made the wrong decision," Gasly added.
Bahrain International Circuit - Winter testing
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