The 65-year-old has already had his access stripped back to minimise the damage to Red Bull if he does choose to join a rival F1 team.
Though Newey's next destination is not yet confirmed, he has been strongly linked with Ferrari to join up with Lewis Hamilton next season.
Where will Adrian Newey go next?
Newey has also been linked with Aston Martin, where double world champion Alonso has recently extended his contract until 2026.
The Spaniard has raced in F1 since Newey's time at McLaren in the early 2000s, and their paths have crossed in rivalry almost continuously since.
Alonso was asked by Agencia EFE whether Newey was his biggest rival, despite the storied on-track battles with the likes of Michael Schumacher, Lewis Hamilton, and Sebastian Vettel.
“Surely, surely!" Alonso replied, laughing. "Because in the years when I was at Ferrari, Red Bull was the great dominator, with Vettel and with [Mark] Webber. Either of them could win you the race.
“And in this era, the truth is that Verstappen is dominating everyone. So Newey is largely to blame for this, yes.”
With Vettel and Verstappen, Newey and his design team masterminded seven drivers' titles, with the Dutchman on track to make that eight this year.
Alonso has never worked with Newey, though is still hopeful of the chance to do so in the second twilight of his F1 career.
“We have a very respectful relationship - mutual," the 42-year-old said of Newey. "He released a book in Spain and gave me the opportunity to write the prologue.
"I think both of us have said publicly that we admired each other; that we greatly respected each other's work. But, apart from that, I think little else.
“He is a fierce rival, who has helped all the teams to be better; to always look for a better version of ourselves. Now that he is free, on the market, logically we would all want, or dream, of having that possibility.
“But, well, also reading the rumours or the things that are said, it seems that he is close to Ferrari. So we'll see. Whatever he decides, he will be welcome.”