In terms of outright competitiveness, the move represents a step backwards for Ricciardo, with Renault unlikely to eliminate the significant gap in performance to Red Bull in the short-term. "I can't help but feel that was perhaps a large part of Daniel's decision making".
"I think Daniel decided after a long flight to America, he decided he wanted a change", says the Briton.
Horner said he could understand Ricciardo's decision to leave "if it was to Ferrari or Mercedes" but said committing to Renault, which is fighting at the front of the F1 midfield as it rebuilds its works team, is "an enormous risk at this stage in his career".
"The competition between he and Max is intense".
Red Bull team principal Christian Horner has ruled out signing Fernando Alonso as Daniel Ricciardo's replacement on the grounds that the two-time world champion has caused "chaos" wherever he has driven.
But a strength can also be a weakness and Horner believes Verstappen's prowess was a critical factor in Ricciardo's shock decision to depart and partner Nico Hulkenberg at Renault next year.
Horner described Red Bull's attempts to re-sign Ricciardo as akin to "trying to convince a girl to go out with you who is being pretty reticent".
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'Daniel said he decided after a long flight to America that he decided he wanted a change, now you look at the rational reason for that and it's hard to understand.
"In the end we gave Daniel everything he wanted and asked for and it still wasn't enough". So I think we are going to sit back and just evaluate what the situation is and also see what comes out of the woodwork because it is an incredibly attractive auto to be driving.
Ricciardo took to social media to thank his fans and Red Bull for a decade of great memories and opportunities, but said he felt it was time for a new challenge.
"We were even prepared to do a one-year agreement should Ferrari or Mercedes come knocking in twelve months' time", Horner confirmed.
"It then became very clear that was his choice and you have to respect that".
So it wasn't about money, it wasn't about status, it wasn't about position or commitment or duration, I think he felt that he needed to take something else on at this stage in his career. "It might be an inspired choice, it might one that he regrets".