To fill Sainz's place, axed Daniil Kvyat will return, but whether the Russian will race full-time alongside Pierre Gasly next year is not yet clear as German Wehrlein, the Mercedes-backed Sauber driver, has been linked with a seat at the junior Red Bull team.
However, Red Bull's Marko told Auto Motor und Sport: "Wehrlein is not on our list."
Asked who is on the list, the Austrian answered: "Perhaps there will be a surprise."
Another rumour is that the McLaren junior Lando Norris is in the running, perhaps as part of the deal that sees Honda power also heading to Toro Rosso in 2018.
Yet McLaren boss Eric Boullier said: "It is one year too early for Lando to come to Formula 1."
The 2017 calendar continues with the United States Grand Prix in a fortnight's time.