"Lukaku is fully fit," he told reporters a day after resting the Manchester United target man and eight other first-choice players but still claiming a 1-0 win over England.
That victory has arguably handed Belgium the harder half of the draw than England but Martinez refused to be drawn on the potential difficulties of facing former champions Brazil in the quarter-final. He said beating an understrength England with his reserve team had filled the squad with confidence.
It also gave Martinez a chance to try out two key defenders who had been out injured for several weeks.
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Martinez said Thomas Vermaelen was still suffering poor vision due to a black eye sustained in a clash on Thursday but would be back to full fitness before Monday's game.
The coach said Vincent Kompany, who played the last 20 minutes, was also now fit, though it was not yet clear if he was sufficiently match-sharp to start against Japan.
(Reporting by Alastair Macdonald, editing by Neil Robinson)
