Last year's Formula 2 champion Piastri will make his F1 debut with McLaren next year after finding himself at the heart of a contract saga in recent weeks involving both McLaren and Alpine.
Piastri rose through the junior ranks as part of Alpine’s academy, but the team was found not to have a valid contract in place with him for an F1 drive in 2023.
The saga was the biggest talking point in F1 over the summer, with Horner previously saying there was “something not right” for Piastri to not have a firm drive in place with Alpine.
During the latest episode of F1’s Beyond the Grid podcast, Horner said it was “difficult” for him to comment on Piastri’s situation, but that contracts would have been watertight had he been under Red Bull’s wing - something there was a chance for a few years ago before he moved up to Formula 3.
“He drove for the Arden team in Formula 4 and Formula Renault, and was obviously a significant talent,” said Horner.
“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.”
Piastri raced for Arden - the junior team founded by Horner - in British F4 and Formula Renault Eurocup through 2017 and 2018. He ultimately joined Renault’s academy at the start of 2020 ahead of his back-to-back title wins in F3 and F2.
Horner said the fallout from Fernando Alonso’s decision to leave Alpine for Aston Martin in 2023 had “created the ripple that rolled out over numerous seats”, which now includes Red Bull’s sister team, AlphaTauri.
After losing the contract case to keep Piastri for 2023, Alpine has turned its attention to signing current AlphaTauri driver Pierre Gasly.
Red Bull will only agree to release Gasly if it has a replacement already lined up. Its plan to sign IndyCar driver Colton Herta was shelved after the FIA refused to grant an exemption for a superlicence, but Nyck de Vries has now emerged as a candidate for the seat.
It is not thought Red Bull is considering any of its existing young drivers for an F1 promotion in 2023, but Horner felt its programme was still in good shape.
“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,” said Horner.
“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?”
