Toro Rosso picked Vettel over big names
Horner says it was a "straightforward decision" to bring the German in.
"Well, after David informed us in Canada that he intended to retire at the end of the season, it naturally triggered a discussion with Sebastian, who is on a long-term contract with Red Bull," he told the official Formula One website.
"It was a very straightforward decision, especially when we took into consideration the potential he has shown in the last eight months.
"There were the means to make it happen very quickly, there was a very short discussion with Sebastian and then, internally, everything was put in place to make his switch happen."
Red Bull, though, held talks with several other candidates, but ultimately opted for Vettel.
"We had a few discussions with 'big names' but we are very comfortable with the pairing of Mark (Webber) and Sebastian," Horner said.
"It is a good blend of youth and experience and Sebastian's potential looks quite exciting. And I hope that we can provide him with the right environment to help his potential flourish."