Wenger is preparing for his 1,000th game in charge of Arsenal this weekend when the Emirates Stadium side take on Premier League leaders Chelsea at Stamford Bridge in a pivotal clash in the title race.
Dein, who helped to bring Wenger to north London in 1996, was speaking exclusively to Sky Sports about the Frenchman who has helped establish Arsenal as one of the most envied clubs in the world.
"He has transformed the club, revolutionised the club," Dein said.
"Whenever he decides to leave he will be leaving behind a phenomenal state-of-the-art training facility, a wonderful stadium, a very good squad which he is trying to improve and a legacy of what he's won.
"Arsene is a football purist. He has a lovely phrase, he calls it 'possession with progression'.
"Arsene Wenger transformed the club on the pitch and off the pitch. I will go on record as saying he won't be a difficult act to follow, in my opinion it will be an impossible act."
With the current trend in modern football being for managers to be given very little time to improve a club's fortunes, Dein does not expect another boss to come close to matching his record.
"I would go on record and say I don't think we will see that again in our lifetime," he said.
"It's a phenomenal milestone; 1,000 games at one club for 18 years.
"In modern football the longevity in the whole of the 92 professional clubs, you know how long a manager stays in his job for? Eleven months, and Arsene has been there for 1,000 games which is absolutely astonishing."
Despite not winning a trophy for eight years Dein believes Wenger's record at Arsenal stands the test of time.
"He's won three Premier League titles and two doubles in that of course," he said.
"Four FA Cups, he is the longest-serving managed in the Premier League today and he is if I'm not mistaken been manager of the month 13 times which is in itself more than anyone else other than Sir Alex Ferguson, so he leaves behind a phenomenal legacy of what he has achieve on the pitch but also off the pitch."
Wenger is yet to agree a new contract - his current deal expires in the summer - creating some uncertainty over his future.
Dein is certain, however, that he has the passion to stay on, adding: "He still has the fire in his belly and wants to win every game.
"We have a ritual that after a home game we go up the road and we normally have dinner together with his wife and my wife - but only if we haven't lost. If we've lost he's bad company. It's a monologue instead of a dialogue.
"The other day I said to him you've probably had 1,000 games with other clubs and 1,000 with Arsenal, you've probably had collectively about 2,000 games.
"I asked what does that mean to you? He looked at me and said: 'Two thousand sleepless nights'.
"That tells you a lot about the man."
"Arsene who?" - a rare insight into Arsene Wenger by the man who brought him to English football - is available On Demand.