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The performance of Lukaka when he is on international duty for his country can not be compared to his performance at the club side because looking at the players in the Belgium national team where Lukaka came from, he always have a good number of players who's assisting him to get the job done and the level of competition in the national team football can not be compared to that of the clubs because the club football is more difficult and the players you play with may not be used to each other since some of them may be new to the team and they will take some time to get used to the performance of each other and that will really have some negative effect on the performance of players especially the strikers.
Overall I agree with you, Lukaku performance when on international duty for his country cannot be compared to his performance when working for his Club. Especially also, Lukaku is still quite new at the Roma club, so indeed he needs more time to have good chemistry with each other in that team. And so far Lukaku performance while working at the Roma club is not too bad, as a new striker he has worked as well as possible. I hope this season Lukaku does not suffer any injury that could reduce his contribution in scoring of goals.