The Italian has impressed while standing in for first-choice Wojciech Szczesny in recent weeks but the Frenchman admits he would have been free to leave the club just months ago
Mannone was sent on loan to Championship side Hull City and, despite impressing at the KC Stadium, the Frenchman didn't appear to see a future at the Emirates for the former Atalanta trainee.
But after injuries to Wojciech Szczesny and his deputy Lukasz Fabianski, Mannone has been thrust into the first team this season and has shown impressive form, conceding just two goals in four games.
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"Szczesny has the No.1 shirt and it is up to the others to win it. He has not lost it but there is competition now.
"When I sent Mannone to Hull, I watched him every single game and all the reports I had were very positive. I was quite surprised no club went for him because I would have let him go."
Mannone's assured displays so far have eased Wenger's goalkeeping issues, and the Frenchman is glad to have several options in that area of the pitch.
"The goalies need competition but also confidence," he continued. "It is a special position where, if you are challenged every time, if you change every time, you cannot be good.
"If Gianluigi Buffon makes a mistake he knows he will play for Juventus the next game and if Joe Hart makes a mistake he knows he will play for Manchester City the next game.
"That's important, but if, over a period of time, their standards drop, and somebody comes in, that's different."