Red Bull received permission to "beef up" the rear wings on both cars at the United States Grand Prix after discovering a crack on Max Verstappen's.
The team spotted the issue prior to qualifying, which resulted in them strengthening the wing on both of their cars.
Although Red Bull aren't sure what caused the problem at this point, they will have a closer look at it before the race.
"There's always something going on, there's never a stress free session," Horner told Sky Sports.
"I don't know whether Max hit something, because there's quite a crack, and nothing on the other car or any of the other wings, so we got permission to beef it up a bit on both cars."
Verstappen and Perez will start Sunday's race first and third respectively, a welcome sight for Horner given Red Bull are keen to take some points off Mercedes in the fight for the Constructors' Championship.
"We've definitely improved the car. It's tough to follow here, so you definitely want to lead into Turn 1," Horner added.
"It's going to be at grandstand start up there at Turn 1, but at least we've got two guys starting at the front of the field."
Video: Check out the sports cars Verstappen drives away from the track!
While Max Verstappen is most known for his performances in a Red Bull F1 car, his passion for motorsport can be seen in what he drives when he's away from the track.