Lukaku, on loan at Inter from Chelsea, returned to action in the Champions League group game last week against Viktoria Plzen - in which he scored - and played in the weekend Serie A clash with Sampdoria, getting on the pitch for 29 minutes across both matches.
"This morning, Romelu Lukaku underwent tests at the Humanitas Institute in Rozzano. They revealed that the Belgian has suffered a myotendinous strain in his left hamstring," Inter said in a statement.
"His condition will be reassessed in a few days."
Lukaku is Belgium's all-time top scorer with 68 goals in 102 appearances.
But an ankle injury towards the end of last season and the hamstring problem prevented him from playing in five of their six Nations League matches as Belgium missed out on a spot in the finals.
Belgium, ranked second in the world, are in World Cup Group F and begin their campaign against Canada on November 27th, followed by games against Morocco and 2018 runners-up Croatia.
