Many fans and pundits believe that Leclerc will really pressurise Vettel for the number one spot at Ferrari, but Horner believes that this pressure will be good for Vettel. Vettel, who won four World Championships whilst under Horner's leadership at Red Bull, will quickly need to establish himself as the quicker driver.
Horner believes that World Champion Vettel will benefit from having Leclerc snapping at his heels.
"Charles is a really exciting talent," Horner said to AutoSport. "For the sport, it's great to see.
"Sebastian's now 31, you've got these youngsters coming through starting to put pressure on the established guys. From a sports point of view, it's fascinating.
"He [Vettel] had Kimi pretty easily covered. He might have to work a bit harder with Charles but he may now find a new level.
"So it might, in turn, be good for him to be pushed even harder."
In the past, some would argue that Vettel performs best when he is dominating and it will be interesting to see how he gets on with a young racer looking to establish himself coming up behind him.