The energy drink company has a firm contract with the 22-year-old, but persistent rumours say that Renault are interested in running him at the French works team.
Indeed, Sainz admitted recently that Renault even made a "good offer" to sign him for 2017, but it was turned down by Red Bull.
Now, there are rumours that not only Renault but also McLaren are interested in Sainz for 2018.
When asked about Sainz's 100% Red Bull contract for 2018, Dr Helmut Marko told Canal F1 Latin America: "It's difficult to talk about 100% in Formula 1.
"I would not be surprised to see him at another team. Because I know what his contract is.
"But only we can make it possible."
The 2017 calendar continues next Sunday with the Singapore Grand Prix.