Toro Rosso boss Franz Tost believes Carlos Sainz owes an allegiance to Red Bull and should consider himself lucky to be in F1.
As a Toro Rosso-Honda partnership looms following a probable engine supply deal between McLaren and Renault, Sainz could join the French manufacturer's works team as part of a compensation package required by the latter in order to end its contract with Red Bull's junior outfit.
Regardless of the ongoing complex negotiations and their ultimate outcome, Tost would disapprove of Sainz's departure.
"Carlos has a contract with Red Bull which financed his career," the Austrian manager told France's Auto-Hebdo.
"And now that he has reached maturity, we should let him go? Red Bull isn't in the business of forming drivers for other teams.
"What if something happens to one of Red Bull Racing's drivers? This is precisely why Red Bull has two teams in F1."
Tost believes the energy drink company's heavy investment in young talent warrants a return, something obviously lost on Sainz says the Austrian.
"They've invested a lot of money and I don't accept a driver waxing on about how "I want to join another team".
"He should be happy to be n Formula 1, especially as the Toro Rosso is a good car..."