Red Bull were plagued by their cars not finishing several races throughout this season.
In contrast, title challengers Hamilton and Vettel drove through the chequered flag at 20 of their 21 outings.
Hamilton won his fifth Drivers’ Championship this season as he beat Ferrari’s Vettel by 88 points.
But Red Bull are hoping to make the 2019 season a three-horse race after switching engine suppliers from Renault to Honda.
Renault were to blame for about half of Red Bull’s DNFs in 2018 but Horner hopes the manufacturer change, along with Verstappen’s newly-found maturity, will serve them well.
“Lewis Hamilton had one DNF this year, likewise with Sebastian,” Horner said.
“That’s the rate we have to be aiming to achieve.
“Reliability isn’t something we’ve had.
“We’ve had 11 or 12 retirements this year. That’s [been] our biggest Achilles heel.”
Horner is glad to see Red Bull heading in the right direction.
But there is still some way to go to catch Mercedes and Ferrari.
“The trajectory is going the right way,” he added.
“It’s always easy to say ifs, buts and maybes but if there’d have been another 40kw in the back of the car, the season could have looked quite different.
“When you look at the other users with the same engine, there is a world of difference.”
Source : EXPRESS