"He comes to the club at the top of his game," Wenger told Arsenal's official website. "It was a very good deal for us at a good moment. I think it was a no-brainer as well.
"With two goalkeepers available for the Premier League and all the competitions we face, we felt we were a bit short because sometimes you have injuries and sometimes you need to rest one of the goalkeepers. At the moment, Wojciech is number one unless I decide otherwise.
"They are all competing for the place of the number one. At the moment, one and two are Wojciech and Fabianski. After, maybe I will rotate. Viviano is a highly-rated goalkeeper in Italy so we have three goalkeepers that are very compact quality-wise."
Viviano played for Fiorentina on loan from Palermo last season.