Wenger, who has been at the helm of Arsenal since 1996, has come under fire from some of the club's supporters in recent seasons, having failed to lead the Gunners to silverware since 2005.
The sale of top players such as Cesc Fabregas and Robin van Persie has also angered fans as the club struggle to challenge for honours.
Dein, who stepped down as Arsenal vice-chairman in 2007, finds the criticism of Wenger "very hurtful".
"People have got short memories. They seem to forget what Arsene has achieved for the club," he told The Sun.
"Arsenal is respected around the world and much of that is because of Arsene with the results and the players he has brought in and developed.
"People underestimate him. He has transformed the club. He has done a phenomenal job.
"I always used to call him the miracle-worker. He waved his magic wand and he delivered.
"But it's getting tougher because the competition is getting stronger."