Belgium scored five goals in their win over Tunisia, but Steve Nicol says their issues at the back could prove costly in the World Cup.
Of Lukaku, who has now scored 17 times in his last 11 worldwide matches, including four World Cup goals, Martínez said: "It's his external ankle ligament of his left foot".
Injuries to striker Romelu Lukaku and his two forward support men did not seem to trouble the Spaniard, but given a lack of recovery time he is planning on giving match time to others in the 23-man squad. We conceded two, so that's the thing we can improve but I think the team deserved to win today now we enjoy the next four days and then we play England for the top of the group.
"Romelu Lukaku is so confident right now", he said as part of BBC's coverage of the game.
Belgium's all-time top scorer became the first player to achieve the feat since Diego Maradona at the 1986 World Cup in Mexico - the Argentina great did so en route to lifting the trophy.
Roberto Martinez's side, who emerged as 3-0 winners in their opening Group G game against Panama, were always two steps faster than the North Africans.
Now with seven goals, the Manchester United striker has scored more goals than any other Belgian player at major tournaments having scored one at the World Cup in 2014, two at Euro 2016 and four in Russian Federation.