"Everything else will fall into place towards the end of the summer. There is no pre-conception," he said, adding that Vettel wouldn't have any 'rubber stamp' on the team decision regarding his future team mate.
"He is a sporting guy and has never pressured the team into doing things for his own benefit," said Horner. "He is going to do the best job for himself. And to say, 'This one, yes' and 'This one, no' was never Sebastian's style [or] to dictate, 'I must have this or that.'"
For now, though, all eyes are on success in 2013, and Horner added that the biggest threat to another championship-winning season for Red Bull wasn't any internal friction between the drivers, but rather problems handling this year's latest Pirelli tyre compounds.
"Whoever masters and understands these tyres best and most consistently will emerge [champions] at the end of the year," Horner said, adding that he didn't expect the tweak that the tyre manufacturer has made coming into Spain to have a significant impact.
"We've managed to win two races out of four on the previous compounds so I think it is not such a huge difference," he said. "It was the first look at these compounds here today, so we will see on Sunday."