Last year, the impressive Toro Rosso driver was linked with a move to Renault, but Red Bull elected to keep him at the junior team and as a 'reserve' for the main outfit Red Bull Racing.
"Carlos has a contract with Red Bull, who brought him into F1 and he's there for the investment they made," Horner, the Red Bull team boss, told Spanish broadcaster Movistar.
"So at the moment we do not intend to release any of our drivers.
"Carlos is developing and the Toro Rosso is a good car that should give him good opportunities. He's still very young but we value him a lot and that's why we don't want to get rid of him."
Horner was also asked about rumours linking Daniel Ricciardo with Ferrari for 2018, but the Briton insisted: "There are always rumours in Italy."
The championship continues next Sunday with the Russian Grand Prix.