Now at 25, and in the prime of his career, the Toffees appear likely to cash in on their record signing and use the substantial transfer fee to replenish their ageing roster.
Having joined the Toffees for £15 million, as a relatively unheralded 20-year-old, Fellaini has slowly forged a reputation as one of the leading midfielders in the Premier League.
He can be destructive and relentless in defence while being versatile enough to offer an imperious presence in attack. However, when his Everton career is fully assessed, it's unlikely fans will revere him as much as some would expect—despite his high reputation in football.
While Fellaini is undoubtedly one of the most high calibre players to play for the Toffees in recent times, he doesn't come close to the category of club great.
First of all, Everton have an especially illustrious heritage. While the club are currently seen as financial overachievers, valiantly maintaining a European push for as long as possible, a few decades ago they were frequently in the title picture.
The Toffees' trophy stash of 15 major honours—including nine league titles—is among the best in the country. It's simply not possible for Fellaini to be remembered alongside the names who helped capture those titles, considering the fact he's won nothing during his stay.
Yet it's not just the elite names of Everton's history who may eclipse him over time; but more recent servants, including the likes of Tim Cahill, Mikel Arteta and Duncan Ferguson. Players who, purely on talent and potential, the Belgian arguably surpasses.
When it comes to the more modern-day heroes, the two factors tarnishing Fellaini's legacy is both his perceived commitment and his consistency.
So much of the Belgian's game is based on reputation and potential. He certainly has the potential to dominate games, but so often a late goal in a poorly covered game will augment his reputation, with few there to report on his overall showing.
That's not to suggest he hasn't at times been a colossus for the Toffees, as he certainly has. However, given his considerable attributes, he should dominate far more often and there are too many instances when his mind inexplicably seems elsewhere.
Everton's farcical recent performance against Wigan in the FA Cup quarterfinal is a prime example. The match was the biggest contest of the Toffees' season, yet Fellaini—Everton's key man and marquee name—produced his most abject performance of the year. He was even booed off after a premature, but understandable substitution.
Fellaini has scored in big games this year against Manchester United and Arsenal but, in truth, he's more often been a passenger in the season-defining clashes. This campaign, the Toffees actually average more points per game without him.
To be fair, some of this is perhaps linked to his positional change. Last season as a deep-lying midfielder he produced his most consistent term in an Everton jersey, but has now returned to an attacking berth where his performances have ranged from the sublime to the irrelevant.
With so many facets to his game it's a crime he doesn't influence almost every game he plays in, something that frustrates many who continually watch him.
Given his main event billing at the club, the fact he's never come close to winning the Everton Fans' Player of the Year award is a telling sign of this inconsistency, as well supporters' overall expectancy.
Leighton Baines, Phil Jagielka, Steven Pienaar and John Heitinga have taken that trophy during his Everton career and, despite his rampant start to this season, his form's again tailed off, making it unlikely he'll ever win it.
Has Fellaini fulfilled his potential at Everton?
That flaky commitment doesn't just appear on the field. In five years at Everton there has been a deluge of controversial comments leaked from the Fellaini camp highlighting his ambition to leave the Toffees for bigger teams. Words that fans find it very hard to forgive and forget.
Most players pledge a love for their club, but the way the likes of Cahill, Arteta and Ferguson handled themselves in and around Everton will see them evoke far fonder memories from fans looking back. It also helps that they had very successful and consistent careers on the pitch and prospered in vital games.
There's no denying Fellaini's talent, but it takes far more to be remembered as a club legend.
He doesn't come near comparisons with trophy winners such as Dixie Dean, Brian Labone, Alan Ball and Kevin Ratcliffe. And while his ability may surpass most Everton players of the last two decades, it's unlikely his eventual legacy will be viewed as highly.