Jelavic scored twice, and missed a penalty, during Everton's 4-0 victory over Queens Park Rangers in the FA Cup third round at Goodison Park on Saturday.
However, the 28-year-old has found first-team opportunities limited so far this season, starting just five Premier League games, and reports have subsequently linked with him an exit to QPR.
He hopes to make Croatia's squad for Brazil and, despite Martinez being loath to allow him to leave, he appreciates that it may be in the best interests of the player.
"I don't want to sell Jela, but I understand it's a key moment for him," he said after the FA Cup win.
"If the World Cup wasn't this summer, this wouldn't be discussed."
Martinez was pleased with by the striker's display against QPR, saying: "I thought Nikica's performance was very good."
Talking of the penalty miss, Martinez added: "He is frustrated he didn't get the hat-trick and he deserved to.
"He hadn't done that in training before. He executes penalties very well, but the chip showed confidence."
Martinez was also sympathetic of the World Cup hopes of defender John Heitinga, who recently rejected move to West Ham, as he came off the bench in the win over QPR.
"We have been very open from the beginning," he said.
"Today you saw two players in Nikica and Johnny who have incredible experience and have been instrumental in our day-to-day working at the training ground.
"They are extremely pristine professionals and I need to listen to them.
"They are frustrated with the lack of minutes with the World Cup around the corner and I need to look into that. That doesn't mean we want to sell them.
"As a football club, if there are any offers or opportunities for the players, we will look into it, but it has to be right for the squad and the club for anything to happen.
"We want to finish the window stronger than we started."