The Red Devils are in pole position at the top of Group G after adding a 5-2 rout of Tunisia on Saturday afternoon to their 3-0 win against Panama. Substitute Michy Batshuayi also got on the scoresheet, so Belgium have a class replacement ready to go.
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".
"It's just the second game, we just need to keep improving like we are doing now".
The record of 18 goals scored via spot kicks at a World Cup finals is shared by the 1990, 1998 and 2002 competitions - the former was a 24-team finals featuring 52 matches played, while the latter two editions came after the expansion to 32 teams and 64 matches. "We will assess them and there will be major, major changes against England".
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. That's what we want.
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They announced their engagement in the Times newspaper past year and are expected to marry at Wardhill Castle in Aberdeenshire. Speaking outside the castle Sebastian Leslie said: "We are absolutely thrilled for Kit and Rose to be marrying today".
"For his second, he dinks it over the goalkeeper with his weaker foot and he's celebrating before it even crosses the line - supremely confident".
Belgium's next game is against Tunisia on Sunday, before rounding off their group with a huge game against England.
With his brace against Tunisia, Lukaku wrote his name into the history books of the Belgian national team.
The three North African countries that qualified for the 2018 World Cup, have been eliminated from only two rounds of matches.