Having struggled with an ankle problem towards the end of the club season with United, Lukaku picked up a knock during Belgium's 5-2 win over Tunisia at the weekend.
Despite reports that he suffered no new damage to the ankle, the 25-year-old sat out training on Tuesday, most likely as a precaution and for rest.
Eden Hazard and Dries Mertens both also suffered minor issues in the Tunisia game and their participation against England could be limited as a precaution, even if there are no major concerns.
"Lukaku needs some time, but there is no damage, I am not worried about Lukaku," Martinez is quoted as saying by Belgian outlet Sporza.be when he addressed the media this week.
"Mertens will be able to train [on Tuesday], we will stay with Hazard," he added.
Martinez is likely to leave Jan Vertonghen, Kevin de Bruyne and Thomas Meunier out of the team to face England, with all three carrying a yellow card and at risk of suspension in the Last 16.
The coach suggested it would be 'unprofessional' of him to risk them in what is essentially a dead rubber given that both teams have already qualified for the knockout stages.
"We have achieved our first goal, now we have to make sure that our team stays at its strongest, of course we want to do well against England, but our team must be equally strong after that match," the former Wigan and Everton boss commented.
Manchester City captain Vincent Kompany could be one of those to come into the team, depending on how he handles training over the next few days.
"From a medical point of view, he is fully fit and he can play. We have to see how he has digested the training, and only then will we decide whether he is playing or not," Martinez said.
In a bizarre turn of events, it may actually be better to finish second in the group, given how the rest of the knockout bracket is shaping up. Both teams should have enough to beat any Group H side in the Last 16, but winning the group would likely mean facing Brazil in the quarter finals, whereas second place probably means playing Mexico or Switzerland in the last eight.
Ultimately, however, any country looking to win the World Cup must beat the best at some stage.