The midfielder is the Toffees' top scorer with eight Premier League goals to his name and, while the likes of Nikica Jelavic, Kevin Mirallas, Victor Anichebe and Steven Naismith have all chipped in with goals, Fellaini has now scored five of the team's last six in the Premier League.
Although Osman feels that the rest of the team need to make an effort to share the goalscoring burden, the recently capped England international also suggests that it is only natural to count on your biggest stars.
He told the club's official website: "It's always important to try and spread the goals but, at the same time, you're going to get a period in the season where it's tough and you need your players playing up the pitch to score the goals.
"Felli's certainly been doing that for us lately. He's a massive player for us at the minute, that's evident in the way he's playing and the way we play.
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Having secured 1-1 draws against Arsenal and Manchester City in the last two league games, Everton will play host to Tottenham on Sunday and Osman is expecting another tricky encounter against the in-form Londoners.
"It's another tough test, of course," the midfielder continued. "Tottenham are one of the better teams in the league.
"They were unlucky not to get a Champions League place last year after finishing fourth, so it's important that we're on our game this weekend for a difficult match."