Red Bull are in "control" of Carlos Sainz's Formula 1 future, according to rival Mercedes's chief Toto Wolff.
When Mercedes were on the market to replace Nico Rosberg, rumours emerged that Wolff looked into Toro Rosso driver Sainz's contract situation.
Wolff told El Mundo Deportivo: "Yes, Carlos is ready and I like him and I like the family.
"Carlos is what he has to be - a modern driver, handsome, intelligent, fast."
It is believed that Renault also looked into signing the 22-year-old for 2017, but Wolff said: "He is a Red Bull driver and they will always try to keep the good ones in their system or to control their destiny.
"And that's the situation right now."
The Red Bull driver programme is headed by Dr Helmut Marko, who told El Confidencial that while Sainz is developing well, he still has room to improve.
"He has gone from being Carlos Sainz junior to Carlos Sainz the Formula 1 driver," he said.
"But I think he would develop better if he left the 'Madrid protection zone' of his father. He is still like a young bird in the nest.
"He is fast - very fast - but he has to be consistent. He made a stupid mistake in Bahrain, but he's young, he can learn, and he has to be patient."
Although Red Bull Racing's drivers Max Verstappen and Daniel Ricciardo are under contract for 2018, there are rumours that one of them might depart anyway.
When asked if Sainz might be released in that event, Marko answered: "Traditionally we talk about drivers after the summer. It's too soon."
As for strictly holding Sainz to his contract, he added: "If it was a much better team than Toro Rosso, we can talk.
"But Ferrari did not ask for Carlos - that was in the dreams of Carlos senior. So far no-one has called us.
"He must concentrate on doing the best job possible and he will automatically develop. On the other hand, we need a reserve driver and Carlos is our first option."
Sainz currently lies in 10th place in the driver rankings with 17 points.
