After scoring 15 times during a loan spell at Goodison Park, the Belgian international agreed a permanent move to the Merseyside club during the summer, but has failed to recapture his goalscoring form of last term.
However, Martinez is adamant that Lukaku's World Cup workload has played a significant part in his dip in performance.
He told the Liverpool Echo: "Rom is not someone who looks for an excuse. He understands that the potential he has as a footballer will always bring huge expectations. He embraces that.
"The situation we have is that he was unable to get a full pre-season after the World Cup and the knock he had so early in the season meant we couldn't allow him to have a proper introduction into work.
"He had to get himself fit by playing games and now we're paying for that."
Everton will return to Premier League action by hosting Aston Villa on Saturday.