The striker has been suffering from an ankle problem in the past week.
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.
Despite playing 90 minutes and scoring late on, Lukaku will sit out Belgium’s penultimate World Cup qualifier away to Bosnia tomorrow.
The Manchester United star has been undergoing individual training away from the group.
And according to the Manchester Evening News, Mourinho wants constant dialogue over Lukaku’s fitness.
The Portuguese is said to have instructed his staff to keep in contact with Belgium’s team doctors during the break.
While he will miss tomorrow’s game, Lukaku could return to action against Cyprus on Tuesday.
Mourinho has apparently had to wait on Roberto Martinez’s final decision.
The Belgium manager will not make a call on selecting Lukaku until 24 hours before the Cyprus showdown.
Belgium have already qualified for the 2018 World Cup in Russia, so Mourinho will be hoping Martinez opts against risking Lukaku.
The £75m summer signing has another week to get prepared to face rivals Liverpool.
