Ricciardo threw the F1 world a curveball last week when he announced that his fifth season with Red Bull would be his last.
The team also have Carlos Sainz, now on loan at Renault, on their books, although Spanish newspaper Marca claim Verstappen is against being reunited with his former Toro Rosso team-mate. "If you look at the rational reasons for that, it's hard to understand but Daniel obviously had his reasons".
"Daniel had his reasons". I think it comes back to wanting to take on, his words, "a new challenge".
"It's not that they were treated in any way different". They would have absolute equal status, as they've always had. "I can't help but feel that was perhaps a large part of Daniel's decision making".
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 him and Max is intense".
"I think Daniel decided after a long flight to America, he decided he wanted a change", says the Briton.
Horner labelled Ricciardo's two-year Renault deal as an "enormous risk" and said Red Bull "bent over backwards" to keep him.
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".
"I remember when Mark Webber left in 2013, it was going to be a disaster and we considered Kimi Raikkonen and we considered all the drivers, but eventually it came down to a clear cut decision between Daniel Ricciardo and Jean-Eric Vergne and Daniel got the nod".
"It wasn't about money, it wasn't about status, position, commitment, or duration".
"I thought he was winding me up to be honest when he rang me to say "I'm going to Renault", Horner admitted.
'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.
'Excited for what lies ahead with Renault but I just want to thank Red Bull racing, Red Bull the company for everything they have done for me, I really appreciate it, from the bottom of my heart'.