The Napoli forward has struggled with a hip injury and has only played 64 minutes for his Italian club this season and has not played for the national team since last June against Wales.
In the away match against Croatia, a World Cup semi-finalist four years ago, Lukaku was substituted on in the 73rd minute and scored the 2-0 goal in extra time.
Youri Tielemans had given Belgium the lead in the 38th minute.
Belgium, who face Tunisia at home on Saturday, play their first match of the World Cup against Egypt on June 15.
Croatia, with 40-year-old Luka Modric in the starting eleven, return to action against Slovenia on Sunday and face England in their first World Cup match on June 17.
