Kaka has been mooted as the 'Mr X' Milan vice-president Adriano Galliani has spoken about signing this summer.
"Kaka is a great player, but he currently plays for Real Madrid," Allegri told Il Corriere dello Sport. "So many players have been reported to be 'Mister X', and Kaka is just the latest. We will see.
"It's clear that if there is to be a 'Mister X' arrival then it will be an important player of value."