The Belgian powerhouse, signed at the start of the ill-fated David Moyes era for 27.5 million pounds, was a convenient scapegoat last season as United endured their worst season in the Premier League, finishing a distant seventh.
There seemed no way back for the rugged midfielder who most United diehards -- spoilt by years watching the likes of Paul Scholes, Eric Cantona, David Beckham and Roy Keane -- thought not fit to sweep the changing room, let alone be put in charge of United's creative department.
Yet, with Louis van Gaal appearing to understand exactly what a fired-up and confident Fellaini can offer his team, the Afro-haired battering ram is beginning to flourish.
Not that the season started particularly well.
He was a substitute for Van Gaal's opening Premier League match, a home defeat by Swansea City that would have revived bitter memories of last season when a large proportion of the home jeers were directed his way.
An ankle injury then sidelined him for a month and when he returned he again found himself benched by Belgium.
Something clicked, however, against West Bromwich Albion last week when Fellaini came on for Herrera at halftime and promptly scored a wonderful goal from outside the area - his first goal for United - and added some much-needed bit to his side's display in a 2-2 draw.
Van Gaal was suitably impressed, so much so that Fellaini started against Chelsea on Sunday and arguably had his best game for United against the best team in the country.
Helping shore up United often-creaky backline at one end, he also displayed his trademark strength at the other end to ruffle Chelsea's back four as United pressed for an equaliser.
That came in stoppage time when he thumped a header that was parried by Thibaut Courtois with the rebound being hammered home by Robin van Persie.
Late goals in "Fergie time" were United's stock in trade during the glory years under Alex Ferguson, but this one was greeted with as much fervour as any.
After some of the humble surrenders of last season when Old Trafford was not so much a fortress but a house of horrors, a 1-1 draw with Jose Mourinho's table-toppers represented a big step in the right direction.
While the balance may not be right and there are still plenty of question marks in defence, the old never-say-die spirit at least seems to have returned.
Van Gaal will know that United are a work in progress, but with so much ground to make up he can hardly overlook any of the club's supposed assets and Fellaini could yet prove to be the signing United thought they had made last season.
Colombian Radamel Falcao and Angel di Maria may be more in the traditional United thoroughbred mould, but Fellaini is the man to do the dirty work needed if the Reds are to reclaim their place in the Champions League.
According to official Premier League statistics Fellaini covered 12.17 kilometres against Chelsea -- more than any team mate -- and made 70 'high-intensity' runs.
It was more like the player who once ripped up the midfield at Everton and Van Persie certainly benefited from the physical presence supporting him in United's route-one attacks.
"@fellainiM was fantastic," the striker said on Twitter. "He did a great job on Fabregas. He is strong on the ball and in the air."
High praise indeed, although it will take a few more such performances of similar stature from the Everton man to completely win over the Old Trafford sceptics.