The news leaves a vacancy at the four-time world champions alongside Max Verstappen, and Horner says he is in no rush to confirm Ricciardo's replacement. I think he felt "I need to take something else on in this stage of my career".
"I think Daniel decided after a long flight to America, he decided he wanted a change".
Or at least, that's what Red Bull team boss Christian Horner thinks, who during F1's official podcast Beyond the Grid spoke about how he isn't a fan of Alonso's reputation for causing chaos within every team he's been a part of.
That included a late request for a one-year deal to give Ricciardo flexibility if Mercedes or Ferrari was interested for 2020, or if Red Bull's switch to Honda engines for next year proved problematic.
"The competition between him and Max is intense". 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 vehicle to be driving.
"I have to admit, it's been a bit like trying to convince a girl to go out with you who's being pretty reticent", he said.
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"Daniel's had conversations with Dietrich, with myself, with Helmut and we have bent over backwards to make it happen, but if your heart is not really in it then.it's just felt like that".
Horner revealed that Ricciardo only let him know about the decision the day before it was announced and said he first thought his driver was winding him up.
Gasly, 22, says he isn't anxious that if he goes to Red Bull too soon, he could end up like Daniil Kvyat without a seat at all. Is this a wind-up for the summer holidays?' But 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, or commitment, or duration". "It might be an inspired choice, it might one that he regrets".
Horner said Red Bull racing had given Ricciardo everything he had asked for' but it wasn't enough to stave off his ambitions to race for another team.
"I think it came down to wanting to take on, in his words, "a new challenge" but I also think that he sees Max growing and growing in terms of speed and strength and he doesn't want to play a support role - for want of a better word because it's not that they are treated in any way different".