The Croatian frontman has netted three goals in six starts for the Toffees since making a deadline day move from Rangers for £5 million.
Since then, the Toffees have secured an FA Cup semi-final appearance at Wembley and climbed to seventh in the Premier League, ahead of Merseyside rivals Liverpool.
"I definitely think he has had a psychological effect," Moyes said.
"He has given the players an all-round lift because we have been badly needing a different type of centre-forward for a couple of seasons."
The former Preston North End manager also expressed delight with the speed at which Jelavic has adapted to life in the Premier League.
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"I think he has shown he has something to prove as well. He has scored in the Scottish league and played in Austria," he added.
"To come into the Premier League is a step up, so he has played as if he has something to show.
"He’s scored a few and missed a few, but I think his general all-round play has helped the team get better. He's a bit of a silent assassin really."Everton are targeting their third consecutive league victory when they meet Norwich at Carrow Road on Saturday.