Actually, the Spaniard is merely 'on loan' to the French team this year, as Red Bull has contractual control of his career, and if Daniel Ricciardo leaves Red Bull for 2019, it will probably be Sainz who is appointed to replace him.
At the same time, while Nico Hulkenberg has been faster this year, Renault seems happy to keep Sainz for 2019.
However, yet another factor could be Red Bull's forthcoming switch from Renault to Honda power.
"We can say that it's unconnected to engine decisions but it's not," Renault boss Cyril Abiteboul said at Paul Ricard when asked about Sainz's future.
Abiteboul said that he is "happy" with Sainz, and the 23-year-old driver also sounds keen to stay.
"It would be a pleasure to stay at Renault," he told El Pais.
"I'm happy where I am. I think this team has a lot of potential and, whatever happens, I'm very proud of how we are doing things here at Renault.
"For me it would be a pleasure to stay."
Renault advisor Alain Prost agrees that Hulkenberg alongside Sainz is currently working well.
"Sometimes in a team there are big differences in the attitude of the drivers, or how they work with the engineers," he said. "But from what I see, these two complement each other very well.
"The important thing is that they go in the same direction and push the team to be the best."
Abiteboul agrees: "The loyalty that we feel from our two drivers is crucial, because we are still building the team.
"We are not yet at the highest level so we cannot afford to have selfish individuals."
Although a Red Bull seat could be looming, Sainz seems patient about waiting to succeed in a yellow car.
"I have my ambitions and I am very competitive. I like to win," he said.
"But currently in F1, there are three teams that are a second and a half ahead of the rest, and we really are the first of the rest.
"We know that it will not happen overnight as we still have a lot of growing to do, and hopefully that happens in the next year or next couple of years. I think there's no hurry either."
The 2018 championship continues this weekend with the Austrian Grand Prix.