The striker has been suffering from an ankle problem in the past week, says the Daily Star.
And there are fears over his condition ahead of United's crunch clash with Liverpool at Anfield next Saturday (12.30pm).
Lukaku travelled to Belgium after being called-up for the current international break.
But he had already undergone a late fitness test ahead of United's 4-0 win over Crystal Palace last Saturday.
And the 24-year-old hitman picked up an injury to his ankle in the contest, although scans showed no ligament or serious damage.