The Chelsea forward, on loan at Inter Milan, has been dealing with a Biceps femoris muscle injury.
He was not part of Belgium’s public training session ahead of the FIFA World Cup.
Lukaku, 29, has missed Inter’s last four fixtures due to the injury issue, and is now expected to sit out Belgium’s first two Group F games at the FIFA World Cup.
He will be absent from the Red Devils’ clash with Canada on Wednesday, 23 November, and Morocco on Sunday, 27 November.
Lukaku has made only five appearances across competitions this season, scoring two goals and providing an assist.
Injury problems have plagued his return to Inter from Chelsea on a season’s loan.
His absence from the Belgian side could be hugely impactful, given he is the nation’s top goalscorer of all time with 68 goals in 102 international caps.
Belgium’s final Group F clash comes against Croatia on Thursday, 1 December.
It remains to be seen if Lukaku will be available for that fixture.
Other attacks to be vying for the central striker’s role in Roberto Martinez’s side are Brighton & Hove Albion’s Leandro Trossard, Galatasary’s Dries Mertens, and Rennes’ Jeremy Doku.
Fenerbahce frontman Michy Batshuayi and Lens striker Lois Openda are the other two options.
Belgium boss Martinez’s future is in doubt ahead of the FIFA World Cup
Belgium’s golden generation is coming to an end.
Kevin De Bruyne, 31, Thibaut Courtois, 30, and Eden Hazard, 31, are all coming into the latter stages of their careers.
Many envisioned the Red Devils would have won a major international trophy since Martinez took over in 2016.
However, they managed a third-place finish at the 2018 FIFA World Cup and were beaten by eventual winners Italy in the quarter-finals of the European Championships in 2021.
According to the Mirror, there are doubts over Martinez’s future as Belgium’s manager.
He was confident of sealing an agreement with the country before heading off to Qatar for the international tournament.
Yet, promises he was made have not been kept, and there is doubt over his job as Belgium’s coach.
All contract talk is halted while Martinez attends to managing his side at the FIFA World Cup.
However, Martinez feels let down by Belgium, with his representative saying:
“Belgium keep pretending they want to continue to work with Martinez, but where is the contract? They have told the media that we have been having talks. That is correct. We have had talks several times. But nothing is happening.”