The Mexican star was tipped to be replaced prior to the summer break, after suffering consecutive Q1 exits and DNF’s at the British and Hungarian Grand Prix.
However, Red Bull opted to retain Perez in the hopes that his performances would turn around, but the Mexican now has just 41 points from his last 12 grands prix.
When asked by Martin Brundle how Red Bull ended up with a weaker driver line-up despite having brought through so many stronger drivers, Horner identified how demanding the team’s system was, and the pressure Perez was under.
"I mean we’ve given a great opportunity to so many youngsters over the period of time, many of who you’ve mentioned there," Horner said on Sky Sports F1 at the Singapore GP.
"The Red Bull system, it does demand results and demand performance and of course Max is delivering, Checo has been underdelivering this year.
"Last year he did a good job or a good enough job to finish second in the championship and be constructors’ champion.
"We’re having to look further down the road, we’ve got some great talent, you know we’ve got Liam Lawson on the bench," Horner added.
"We’ve got Isack Hadjar in Formula 2 that’s been until recently leading that championship, we’ve got a very exciting young talent that I’m particularly excited about in F3, Arvid Lindblad, so we’ve got depth in our junior programme."