Sainz was linked to Renault earlier this season but Red Bull quickly took up its option on the Spaniard for next year, saying he would stay at Toro Rosso. Renault then moved on to targets such as Stoffel Vandoorne and Sergio Perez but missed out, leading to the French manufacturer - which will supply both Red Bull and Toro Rosso with power units in 2017 - enquiring whether a deal could be done with Red Bull.
However, Tost doesn't see a compelling reason for Sainz to be allowed to leave, saying he is needed to spearhead Toro Rosso's driver line-up next season.
"Carlos has a contract with Red Bull, and I don't think Red Bull will let him go to another team considering the investment it has made during his career since the first steps in single-seater," Tost told Motorsport.com.
"Today he is a high-level professional, and I see no reason why he could be let go. Toro Rosso needs Carlos: he's fast, talented, and in 2017 we will need a driver of his level."
And Tost also confirms the second seat at Toro Rosso is a straight fight between Daniil Kvyat and Pierre Gasly, hinting he would prefer to retain Kvyat's experience ahead of new regulations but with the latter leading the GP2 championship with four races remaining.
"There will be a noticeable change: wide tyres, new aerodynamics, and for a team like Toro Rosso it will be important to have at least one experienced driver. The ideal would be two.
"It will be important to have an experienced driver like Sainz, then we'll see if Daniil Kvyat or Pierre Gasly will be alongside him. I see no other solution."