Dein insists the Frenchman, who led Arsenal to seven major trophies before their recent drought, deserves nothing but praise for the job he has done in north London.
“One should never, ever forget what he has done for the club," said Dein, the club’s former vice-chairman, to 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.”