i understand the issue of his salary but if he is coming for free then its ok, and he wasnt injured for long time, he said it by him self RM inform him that he is not needed anymore and freeze him out.
i dont think he will be a starter, but lets say maybe allegri has plans for him, any way he is a good add for the team (28 old and had top level experience and very technical player)
for a tectical coach like allegri you need options and players with different qualities and abilities to help in various kind of games, and he will be good card for allegri to play.