Arsenal greats Lee Dixon, David O'Leary and Alan Smith believe record-breaking Arsene Wenger is the last manager of his kind.
On Sunday at West Brom, the Arsenal manager will oversee his 811th Premier League fixture, surpassing the record previously held by Sir Alex Ferguson and at a time when the profession has never been so demanding.
The Frenchman was appointed to his role in 1996 and in the 21 years since has become one of football's most respected figures.
He remains under pressure to deliver a top-four finish after years without Arsenal convincingly challenging for the Premier League title, but former Leeds and Aston Villa manager O'Leary believes he is the last of his breed.
"It's remarkable, amazing, unbelievable," the Irishman told Press Association Sport.
"To do 20-plus (seasons) as a manager: it'll never happen again at Manchester United, Sir Alex was unique, and it'll never happen at Arsenal, Arsene's achievement there is unique."
O'Leary, 59 and a title winner with Arsenal in his playing days, added: "He's had a very understanding board you'd want to be working for. They're not reactionary people, and that's played a part in him achieving that success.
"I came back with Leeds to Arsenal, and if somebody said, 'He'll still be manager of that club', I'd have said, 'No chance'. It's amazing longevity, and at a big club like that. I thought playing was hard, but when you're the manager of a big club, the demands that come with it: you don't realise it until you manage."
The second longest serving Premier League manager is Eddie Howe, who has been at Bournemouth for little over five years but is only in his third season in the English top flight.
"I don't think it'll ever happen again: the likes of Sir Alex and Arsene are a dying breed, and going to be a thing of the past," Smith, 55, also a title winner with Arsenal, told Press Association Sport.
"Managers are getting sacked after four games in the Premier League, so patience has worn more than thin.
"To retain that work ethic and desire is amazing. Whatever you say about the rights and wrongs of him still being in the job, you can't take away from that achievement.
"He's taken more on his shoulders than Sir Alex did. Sir Alex delegated more, spent time overlooking the training ground rather than being down there. A succession of assistants slowly changed training, different voices, different methods. Whereas Arsene, he's had Pat Rice and (Steve) Bouldy, but he likes to oversee everything: that's extra pressure."
Dixon, 53, was a regular in the team that won two doubles under the 68-year-old Frenchman, and he added: "The fact he's reached that milestone is incredible. He must be immensely proud.
"That style of manager, the longevity: it's different to managing a smaller club. When you're at a big club you're under scrutiny every single day, every single game, and sometimes every single training session. The wear and tear on managers in the game that long is immense."