Max Verstappen's growing status within Red Bull was "probably" a factor in Daniel Ricciardo's decision to leave, team boss Christian Horner admits.

The Australian surprised many at the start of the summer break when he put in his notice at the Milton Keynes-based outfit and announced a move to Renault for 2019.

It was such a shock because barely a few days earlier, speculation was Ricciardo had agreed a deal in principle to stay on for another year before changing his mind upon leaving Hungary.

"We looked at it and I think we did everything that we possibly could to retain him," Horner told Autosport in reflection.

"I think he just genuinely wanted to take on a new challenge. He actually said this, he did the classic break-up: 'It's not you, it's me!'"

Even so, amid a tough season which had started so well with two wins in the first six races, it didn't stop speculation that Verstappen's continued improvement and Red Bull's commitment to the Dutchman was playing of Ricciardo's mind.

"I think he was also probably concerned about his value with the evolution of Max," Horner accepted. "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 and yet 12 months ago Max was having that bad luck."

Also Read:

• Webber's message to Ricciardo: Red Bull-Honda could be 'special'
• Ricciardo 'didn't care' about Verstappen comparison as stats hid the 'true picture'
• Ricciardo: Process of making 2019 decision likely impacted my performance

By the end of the year, Verstappen had dominated Ricciardo in the Drivers' standings beating him by 79 points with a total of 11 podiums to Daniel's two.

Horner was keen to stress, however, the situation has remained equal between the two.

"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."