Yet as the temporary-liveried RB14 was revealed and shaken down on Monday, Horner said: "If we've learned the lessons of last year, and if reliability improves, then I see no reason why we shouldn't be on par."
Horner said that a championship for Dutchman Verstappen is "only a matter of time".
"Sebastian Vettel won four titles for us before he was 30. Lewis did his four and is only 32. For me, Max is a champion," he is quoted as saying by Speed Week.
"You can see which drivers are outstanding and Max is certainly one of them."
Alongside Mercedes and Ferrari, Red Bull will have another benchmark in 2018 after McLaren switched to use the same customer Renault engines.
Horner said: "McLaren is a great team and we look forward to racing against them.
"But we made great progress especially in the second half of last season and we hope to take that momentum into 2018.
"The rules are more or less the same, so there are many lessons we can draw from 2017 with the development of our new car."
Red Bull finished third in the 2017 constructor championship.