Manager Roberto Martínez revealed after the match in Moscow that the Manchester United striker suffered "external ligament damage" to his left foot and will be looked at by doctors in the next 48 hours.
If Lukaku, Eden Hazard, Kevin De Bruyne and the rest are to become Low Countries legends, they now have to focus to overcome stiffer opposition than Panama and Tunisia and avoid coming apart under pressure as they did in losing to Argentina and then modest Wales in their last World Cup and Euro quarter-finals.
He scored twice in Belgium's 3-0 opening Group G game against Panama and needed just 45 minutes against Tunisia to score another double.
"That's the most important thing".
Lukaku was subbed off for fellow United man Marouane Fellaini in 59′ due to an ankle issue.
"Football has a odd knack to punish you if you don't treat things professionally and in the right manner", he said.
Speaking in an interview with Sky Sports, Hazard said: "As the captain, even if I am not the captain I can talk, on the pitch I try to give my best, if I can help players to be better - I will".
Hazard and Lukaku both scored braces to secure a 5-2 win in the highest-scoring match of the World Cup so far.
Lukaku's second score was an absolute masterpiece.
"I'm delighted for Eden to come to the forefront as well because we need players like him to perform".
The midfielder scored twice to help his side take victory in Group G, with the Belgians nearly certain to qualify and will if England takes victory against Panama tomorrow.