Jelavic has scored five goals in 10 games since his move to the Merseyside club from Rangers in January and Moyes has hailed the Croatian's impact.
Moyes said: "I do think he's made a big difference.
"He's given everybody the feeling that we can score goals. That was not the feeling at the start of the season.
"We've not had a boy like him, like the way he plays, with his style. We've needed a good centre-forward and he has given us a real boost."
Everton face rivals Liverpool at Wembley on Saturday in an FA Cup semi-final.