Wenger has been linked with a shock move for the former World Player of the Year.
And he told Sport: "Everyone said that Kaka was finished. Now players are judged by their recent performances. When such players are injured for so long, it is very difficult to recover their form if you do not leave a sufficient time.
"I felt last year at the end of the season his form was encouraging."
Wenger added: "Kaka's problem is he's playing in a team that has (Mesut) Özil, who is young and strong. Becoming a first-choice at Madrid will be very hard for him."