The striker left United after two years this summer to join Antonio Conte at Inter Milan in a £74 million deal.
Lukaku has made an instant impression in Italy after scoring on his debut in a 4-0 win over newly-promoted Lecce at San Siro.
His international manager Martinez believes that the move came at the right time for the former Everton star.
“Romelu needed a new challenge,” he said, as reported by the Mirror.
“Sometimes you get bored of a club.
“Working with Conte will please him and the project of Inter, who will chase the title, is interesting. I saw Lukaku laugh again and it made me happy.”
Lukaku is likely to be named in Martinez’s Belgium squad for the Euro 2020 qualifiers against San Marino and Scotland next month.
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