Cech completed an £11 million move to the Emirates in June, with the 33-year-old calling time on his nine-year spell with the Blues.
Flamini admits that he was shocked by Mourinho's decision to allow Cech to leave and believes it could yet come back to haunt him, with the former Blues shot-stopper set to face his old team-mates once again in Sunday's Community Shield.
"I was surprised by Mourinho because he really doesn't do that," he is quoted as saying by the Daily Telegraph.
"He's a competitor and usually doesn't like to sell any players to his rivals, but it's a good favour for us and now we have to take advantage. That's why we have been lucky on this one.
"It's very important. Not only for his quality but also for his character. When you play at the top level, the leaders on the pitch make the difference. Not only when things go well but especially when things go bad. So, for us, Petr Cech is a top signing. He is someone with character, someone with a winning mentality."
Flamini initially left Arsenal in 2008 before returning five years later and the Frenchman believes the club has progressed immeasurably since his first departure.
"I can feel Arsenal now is different to the club I left in 2008. I don't think any of the key players left in the last two years. It's a great example," he added.
"And each year we have added quality players like Mesut Ozil, Alexis Sanchez and Petr Cech. These are players who can make the difference at any moment. So I think we have a very good chance."
