Red Bull team principal Christian Horner says that Daniel Ricciardo's decision to leave the team at the end of 2018 in favour of a move to Renault had come as a complete surprise.
He insisted that he and Red Bull motorsports consultant Dr Helmut Marko had done everything possible to keep the Australian at Red Bull next season, but that ultimately there was nothing to be done.
"We looked at it and I think we did everything that we possibly could to retain him," Horner told Autosport magazine this month.
"I think he just genuinely wanted to take on a new challenge," Horner added. "He actually said this, he did the classic break-up: 'It not you, it's me!'"
Ricciardo had apparently been unsettled by how his young team mate Max Verstapen was becoming increasingly key to Red Bull's long-term planning, leaving him feeling on the outside looking in.
"I think he was also probably concerned about his value with the evolution of Max," Horner agreed. "I think it's been a difficult season for Daniel. He's had to cope with Max's continued evolution.
"While issues have happened to both drivers, they've tended to happen to Daniel on a Sunday rather than Max," he admitted. "And yet 12 months ago Max was having that bad luck."
Horner strongly denied that Ricciardo had been a second class citizen at the team, even after he announced he was quitting for Renault.
"There is zero favouritism in the way that we operate the team, from one driver to the other," he insisted. "Daniel will tell you that very openly.
"The number one driver in this team is the driver who is ahead on the track. That's the simple rule."
The decision to break away from engine partners Renault in preference to the still-unproven Honda power units was also a big concern for the Australian.
"[He] was probably uncertain about Honda at that time," Horner admitted. "Subsequently he's seen what we were talking about start to come to fruition."
Although Horner has been expecting to ink a contract extension with Ricciardo right up to the last minute, he quickly recovered from the shock and had contingency plans in place.
"You always have a Plan B," he said. "Once we understood that he was serious - that that was his mindset - it was a very easy decision to come to regarding Pierre Gasly."
Gasly will move up form the junior Toro Rosso squad to join Verstappen at Red Bull in time for the season opener in Melbourne, Australia in March.
Toro Rosso will have an all-new line-up for 2019, with Alexander Albon the final driver to confirm his seat next season where he will be joined by F1 returnee Daniil Kvyat at Faenza.
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