The Belgium striker completed a protracted club-record transfer to Inter from Manchester United on Thursday for a fee understood to be €80million (£73m).
He is considered a direct replacement for Icardi, who mutually agreed to withdraw from Inter's recent pre-season tour of Asia as he continues to seek a new club.
Icardi, linked with Juventus and Napoli, had played with the number nine on his back since arriving at the Serie A side in 2013.
Diego Forlan, Samuel Eto'o, Julio Cruz, Hernan Crespo, Ronaldo and Ivan Zamorano have also had the same shirt number over the past two decades.
Lukaku could make his Inter debut in Saturday's pre-season friendly against Valencia.