The Frenchman has received criticism from some Gunners fans for their lack of silverware in recent years, with the club's last trophy coming in 2005.
However, Dein, who brought Wenger to the club in 1996, says that the current boss will never be forgotten for his "phenomenal" job in North London.
"One should never, ever forget what he has done for the club," Dein told talkSPORT. "He has absolutely transformed the club. The fact there is a magnificent state-of-the-art training ground, the fact the stadium is virtually paid for.
"He has got a good squad. Obviously it has to be a squad that is capable of winning trophies, and that is something he is going to be working on, I know, but it is not easy.
"But he has done a phenomenal job for the club. He is not going to be a difficult act to follow, he is going to be an impossible act to follow."
Arsenal progressed to the fourth round of the FA Cup on Wednesday with a 1-0 win against Swansea City at the Emirates.