Marouane Fellaini is the most physically dominant player in the Premier League, and perhaps the most dominant player, period. Unlike any other player, his ability to win the ball in any attacking, defensive or transitional situation due to a combination of skill, determination and otherworldly athletic ability, can hardly be matched.
Yaya Toure can cover more ground, Demba Ba can create more goals for himself and Wayne Rooney can pick out his teammates with more precision, but Fellaini possesses the unholy combination of Toure's athleticism, Ba's nose for goal and Rooney's work rate and vision, plus some additional height and hops.
If he was missing one of these traits, he might play somewhere else on the pitch. Without the vision, he'd be a central defender. Without the range, he might be a target man. Without the finishing skills, he might be a defensive midfielder. Instead, he's an unorthodox No. 10, playing as the world of top-level football's only half-midfield wrecking ball, half center forward operating as an attacking midfielder. He's the world's best target trequarista, or its best defensive anchor trequarista, or both at the exact same time.*
*And he has to be, because Nikica Jelavic contributes literally nothing to the team but finishing first-touch shots from inside the penalty area, while Darron Gibson is a glorified energy man whose main asset is working hard for 90 minutes.
Fail to close him down at the edge of the penalty area, and he's capable of placing a perfect, unstoppable 20-yard strike to the far post, through multiple defenders, like he did to secure a draw for Everton against Arsenal. Despite looking much more like a central defender than anything else in his build and movements, Fellaini had the touch, skill and confidence in front of goal to unleash this beauty.
He scored again in Everton's weekend match against Manchester City, though in a much different fashion. On a cross into the box from Leighton Baines, Fellaini made a perfect run to the back post and won a header easily, forcing a brilliant reaction save out of Joe Hart. The ball bounced back to him and he instinctively improvised, netting a rare thigh goal.
While Fellaini's two most recent goals were very different, they were very similar -- much like his overall performances in the City and Arsenal games were very similar -- in that they came in 1-1 draws against supposedly superior opposition. Manchester City and Arsenal have deeper squads than Everton, and spent more money to assemble them (in City's case, hundreds of millions of pounds more). The Toffees were not horribly overmatched in each game, but overmatched nonetheless, and needed the only player on their team (save for Baines, thanks to the worldwide shortage of decent left backs) who could get into the City or Arsenal XI immediately to score a goal that no one else on his team had the skill or athleticism to score.
And therein lies the problem with Everton as a whole. They've impressed in the league so far and have earned their sixth-place standing. They're going to displace one (or two) members of the 'Sky Six' this season if Fellaini stays healthy throughout, because the physically dominant and well-rounded nature of his game means that he's extremely difficult for opposing managers to tactically neutralize.
But if Fellaini doesn't stay healthy throughout the year? There's every reason to believe that Everton will struggle mightily, and it wouldn't be stunning to see them drop out of the top half if he misses an extended run of games. In the two games he missed this season, the Toffees looked a bit lost and drew against Queens Park Rangers and Norwich, at a time when both sides were well in the drop zone and looked fairly hopeless. One can only guess at what might happen if a Fellainiless Everton side came up against a solid, mid-table or barely top-half Premier League side, but the results probably wouldn't be pretty.
This is, after all, a team that is now built entirely around Fellaini. On one hand, it's going to be a recipe for disaster if Fellaini is out for any extended period of time. While Steven Naismith, Darron Gibson and Steven Pienaar have been very solid additions to the Everton side, they're all complimentary pieces who assist a team's attacking centerpiece -- in this case, Fellaini. They can't carry Everton by themselves.
On the other hand, what David Moyes has done is admirable. Fellaini is far and away the most talented player on his side, and one of the five or so most talented players in the Premier League. He happens to play on a team that is far from flush with cash, and only truly financially stable after offloading Jack Rodwell to City for a heck of a lot more than he was worth to literally any other team in world football. While building a team that can withstand an injury to any individual player is the prudent thing to do, prudent is often just a euphemism for "being a huge sissy".
Fellaini wins just about every aerial duel he's presented with. Often, in duels when other players would do well to simply get their head to the ball first and nod in the general direction of a teammate, Fellaini is able to easily get in front of his defender, chest the ball down, and either pass or shoot with his second touch. Whenever his defenders are in a bind, instead of kicking the ball to nowhere and giving back possession out of desperation (or a lack of ideas), they can simply kick it in his general direction, knowing that there's a very good chance that he's going to jump-start an attack from nothing when only a handful of other players on earth would have had a chance to do the same.
Additionally, because opposing teams know that Everton is always looking towards Fellaini and because he's so physically dominant, he clears out space for everyone else. Even when a great player is closing him down, additional defenders have to, at the very least, keep an eye on him. This opens up space for Pienaar, Jelavic and Naismith, who are often made to look a heck of a lot better than they are because they're getting wide open space that they simply wouldn't get if they were playing with an attacking midfielder who was literally anyone else that Everton could conceivably obtain.
Everton will crumble if Fellaini goes down, because he's everything to the team, and supporters of the club should probably be okay with that. After all, the difference between 7th (where a conservative, balanced Everton might finish) and 12th place (where this Everton minus Fellaini might finish) is just a bit of pride and prize money. Everton have a chance to flip the script on the Premier League and finish in 4th place this season because they've built their entire team around their best player.
Moving Fellaini to attacking midfield and building the entire team around him was a brilliant move, and Moyes should be applauded for it.
That is, until Bill Kenwright sells him to Chelsea in January.