Spaniard Sainz has already denied persistent rumours that he could be the subject of a swap deal involving engine price between team owner Red Bull and Renault.
Tost is quoted by Italy's Tuttosport as saying: "Carlos has a contract and I do not think Red Bull is planning to let him go, especially considering his development."
"[Sainz is] a professional of the highest level, and in 2017 we need a driver like him.
"Considering all the [rule] changes we will have, it will be essential to have an experienced driver, and ideally two."
The 22-year-old's current Toro Rosso teammate is Russian Daniil Kvyat.