The Australian has made the move to Renault for the 2019 season, shocking the majority of the paddock, including Horner.
There had been plenty of discussions between the Austrian team and the Honey Badger, and Horner believes they did everything they could.
“We looked at it and I think we did everything that we possibly could to retain him,” he said.
“He actually said this – he did the classic break-up [line] – it not you, it’s me!
“And I think he just genuinely wanted to take on a new challenge.”
Horner also believes that the Australian had some concerns within the team, including the integration of the Honda engine.
He also thinks that the emergence of Max Verstappen had something to do with it, with the Australian admitting that he stopped caring about the intra-team stats halfway through the season.
“I think he was also probably concerned about his value with the evolution of Max [Verstappen], and was probably uncertain about Honda at that time,” Horner added.
“Subsequently he’s seen what we were talking about start to come to fruition.
“You always have a Plan B. I think once we understood that he was serious, that that was his mindset, it was a very easy decision to come to regarding Pierre Gasly.”
Horner also insisted that there was no favouritism within the team, and that the young Dutchman had no preferential treatment.
“The number one driver in this team is the driver who is ahead on the track. That’s the simple rule,” he added.
“And there is zero favouritism in the way that we operate the team, from one driver to the other. Daniel will tell you that very openly.
“Mexico was the first time that Daniel outqualified Max since Monaco.
“Max’s qualifying performances have been huge, and that was probably part of Daniel’s decision taking for the future.”
Horner also acknowledged the Australian’s wretched luck, but also noted that Verstappen had the same issues at times.
“I think it’s been a difficult season for Daniel,” he admitted.
“He’s had to cope with Max’s continued evolution, and while issues have happened to both drivers, they’ve tended to happen to Daniel on a Sunday rather than Max.
“The clutch bearing that Daniel had fail in Sunday in Mexico, Max had on Friday. In practice in Brazil Max’s oil tower developed a leak and a crack that was there from Mexico, and thankfully it didn’t stop the car there.
“And yet 12 months ago Max was having that bad luck.”