Cech joined Arsenal in a deal worth £10million earlier this summer and midfielder Flamini believes the acquisition of the veteran gloveman can only serve the north London club's title hopes positively.
"I was surprised by Mourinho because he really doesn't do that," the Frenchman told 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. It's a top signing. 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."