"I think to go in with two such green drivers is a mistake," said the team principal at Red Bull's Milton Keynes factory on Friday.
"Formula One is a totally different animal to any other formula.
"He has spent 10 years in the sport and throughout that time with top teams, Williams and McLaren.
"The experience that he brings is invaluable at the end of the day, with regards to development and the way that the team operates, and that is obviously something that rookie drivers simply don't have."
Horner, 31, was involved in failed negotiations with Irish entrepreneur Eddie Jordan to take over the Silverstone-based team last year.
While Horner was appointed by Red Bull owner, Austrian energy drink billionaire Dietrich Mateschitz who bought Jaguar Racing from Ford in November, Jordan was sold last week to Russian-born Canadian businessman Alex Shnaider.
Jaguar finished last season in seventh place, scoring twice as many points as ninth-placed Jordan.
Jordan announced on Thursday that Karthikeyan, India's first Formula One driver, and Monteiro would be their line-up for the season starting in Melbourne on March 6.
Red Bull signed Coulthard in December but are in no hurry to announce their second driver, a straight choice between Austrian Christian Klien and Italian rookie Vitantonio Liuzzi.
"There won't be a decision until very shortly before Melbourne," said Horner, whose team and Minardi are now the only ones with seats still to fill.
"Both drivers are under long term contracts to the team and both are heavily involved in the test programme between now and Melbourne.
"(Manchester United boss) Alex Ferguson wouldn't release his team sheet six weeks in advance and the drivers are one of the smallest issues we have. We've got a very good driver line-up in David leading the team and two good youngsters."
Liuzzi won the F3000 title for Horner last year, with Red Bull backing, and could fill the Friday third driver slot whereas Klien is too experienced for that role.
"Tonio is definitely a prodigious talent. It was very evident whilst he drove for me last year," said Horner.
"However you do need to be responsible with talent and progress into Formula One.
"With Christian, he had a generally positive rookie year and he'll have learned a lot of lessons from that rookier year so we'll see."
Horner said fitness was not an issue for either.
"Tonio has been working flat out on a training programme and he's made enormous progress from last November or whenever he first drove the car to today," added the team boss.
"He has done race distances, in Valencia last week, without really any issue.
"He's not at peak fitness but these young guys develop very quickly. So it (fitness) wouldn't be an issue. Both of them are training very hard and will be in the best shape we can have them in for Melbourne."