Red Bull team boss Christian Horner has admitted Carlos Sainz remains the "safety net" should Daniel Ricciardo decide not to renew his contract at the end of 2018.

The Spaniard recently joined Renault from Toro Rosso from the United States Grand Prix and will remain with the Enstone outfit next season as part of the engine deal which will see the French manufacturer supply McLaren and Honda switch to the junior Red Bull team.

He settled in very quickly at his new team too, finishing seventh in Austin, though would fall victim to an engine failure last weekend in Mexico, and his impact didn't go unnoticed with Renault advisor and four-time champion Alain Prost praising the 23-year-old.

"It's always a small risk that you take when you change a driver in the middle of the season, but in fact it was exactly what we wanted to have," the Frenchman told Motorsport.com.

"We had, unfortunately, a problem with Nico and we had Carlos to get points (in Austin). If you want to build something, you cannot have a big hole somewhere. It's not being rude with (former driver) Jolyon (Palmer), but we are constructors and we need to have everything not perfect, but better, and we know we have a lot to do still.

"We are not going to develop [the car] very much, so we want to have the two drivers doing more or less what they have done."

Well aware of the potential Sainz has, however, Red Bull is not willing to let him go and would want him to replace Ricciardo should the need arise.

"[Ricciardo leaving is] a possibility, but that’s why we’ve kept Sainz on loan, and that’s why we have options around us," Horner explained. "We have a safety net, but our number one objective is to retain the pairing we have."

Following Max Verstappen's decsion to sign an extension until 2020, which has caused some rumblings of favouritism within Red Bull, and with the seats available elsewhere, the Briton also acknowledges the Australian is at a vital part of his career.

"The next move that he makes at 28 years of age is a very important one for him, so of course he is going to take the time to make sure the decision is right for him," Horner said.

"He knows the intention very clearly, and I’ve discussed it with him, is that we want him within the team. If that takes another six months then so be it."

That could cause some unhappiness at Renault, as they continue to try and build a team capable of fighting for championships but Prost insists they were willing to accept that when agreeing to take on Sainz for next year.

"In this situation, we don't have all the keys, but the market is very open next year," he said "There is no reason why we should change, but because we don't have all in our hands, we have to wait a little bit.

"We are lucky to get Carlos because we wanted him, but we don't have him for the long term, so we have to be careful. For us, that's not a problem. We are in a period where we are building, so the timing is okay. We want to establish a strong relationship with him."