The 32-year-old Lukaku, who has netted a record 89 goals in 124 internationals, has elected to work on his fitness and will skip Saturday’s clash against the USA and another friendly against Mexico.
"Romelu has chosen to focus on training to further optimise his physical condition," the Belgian Football Association said in a statement on Tuesday.
Lukaku has struggled with injury this season and played only sparingly for his club Napoli.
His last game for his country was a 4-3 win over Wales in World Cup qualifying in Brussels in June, in which he scored. He missed Belgium’s next six internationals.