The Australian was in the middle of a tug-of-war between Alpine and McLaren with the Woking outfit ultimately coming out on top after a verdict was reached by the contract recognition board.
Piastri has enjoyed an incredible rise to F1, winning the Formula Renault Eurocup, F3 and F2 titles in consecutive seasons.
Speaking on F1’s latest episode of the Beyond the Grid podcast, Horner commented on Piastri’s situation and how he had the opportunity to sign him several years ago.
“He drove for the Arden team in Formula 4 and Formula Renault, and was obviously a significant talent,” Horner said.
“There was an opportunity for Red Bull to look at him at the time, and we didn’t take up that option, which is something that I regret. But what he went on to achieve is phenomenal, in Formula 3 and Formula 2.
“Now if he’d have been a driver here, there is no way that he wouldn’t have been under lock and key for a period in time. As I say, I wasn’t party, it’s difficult to judge what was promised or reneged on or so on. But certainly, it was unexpected, probably from several areas.”
Red Bull have promoted a wealth of talent to F1 through its sister team AlphaTauri (formerly Toro Rosso) with the likes of Sebastian Vettel, Max Verstappen and Daniel Ricciardo coming through the ranks.
Colton Herta was being considered but due to issues surrounding him getting a superlicence, that is unlikely to happen.
Nyck de Vries is a potential option for AlphaTauri, particularly after it was revealed that the Dutchman met Red Bull consultant Helmut Marko last week.
While a lot of Red Bull’s F2 junior drivers have underwhelmed, Horner said that he’s been impressed by Japanese driver Ayumu Iwasa in F2 and Isack Hadjar in F3.
“We’re continuing to invest in youth, we’ve got some great youngsters in the programme, all the way from karting and stepping into Formula 4 now, upwards,” Horner added. “I think it’s something Red Bull has done so well, giving these kids a chance, investing in young talent, and giving opportunities they otherwise wouldn’t have had.
“Isack Hadjar in Formula 3 this year has had a great debut season. I think [Ayumu] Iwasa in Formula 2 has been a standout driver for me. They’re just two of the guys on the programme.
“Liam Lawson has had a tough year, but he’s again another talent. Dennis Hauger has been a bit underwhelming in Formula 2 after such a dominant year in Formula 3. But again, how much of that is him and how much of it is equipment?”