The Belgian was a notable absentee as Ole Gunnar Solskjaers men trained in front of a 12,000-strong crowd at the famous WACA Ground in Perth.
The forward has been heavily linked with a move to Inter Milan and is believed to be keen on a move to the San Siro in order to work with Antonio Conte, who he described as the best coach in the world in June.
Inter Milan officials are believed to have met with United representatives in London on Wednesday and Lukaku was conspicuous by his absence in Perth today.
United confirmed that Lukaku, along with Anthony Martial, was doing alternative gym-based training away from the cameras and that he was carrying a little problem.
The Red Devils take on Perth Glory on Saturday in their first game of pre-season and it remains to be seen whether Lukaku will be involved.
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