Not only did Fellaini provide a spark, he also produced the performance of his life, which will not be topped by many Premier League players this season.
The Afro-sporting Belgian completely dominated the United midfield and swatted most players aside as if he were playing against a bunch of kids.
Forget about faceless statistics focusing on his pass-completion percentage, etc. I only care about the eyeball test, and Fellaini passed with flying colours.
Fellaini was all over the place, winning tackles in his own half before storming upfield and carving out chances for his teammates.
A 6'4" midfielder is become less of a rarity in today's game, and Fellaini proved he isn't all brawn and no brain when he expertly dribbled past Antonio Valencia and Michael Carrick to cap a fine mazy run with a shot hitting the post.
And, of course, he finally got the better of Caarick when he scored a towering header and the winning goal from a corner.
But it wasn't just the height and aerial strength of Fellaini that made him such a tough matchup.
Those are nice physical attributes to have, but it was his touch, vision and refined technical ability that made him unplayable on that night.
"If I have got to go, I will go, if not, I repeat, I am at Everton,' the 24-year-old told Belgian radio station RTBF.
"I am ambitious; I want to play in the Champions League or in the Europa League. Last season we finished seventh with Everton, and we were not able to qualify us for Europe, which was a shame.
"If an ambitious club comes in for me? Why not, just to gain experience but I’m on vacation right now, I do not give [speculation] too much attention.
"Every year they say I’ll be transferred to the left or right. But I’m still at Everton!
"If it comes, I take, if it does not come, I stay at Everton, where I am happy, and where I still have a four-year contract.
"Everything goes fast in football. You can be transferred tomorrow, as you can be in one or two years. For now, I’m on vacation, I try to arrive fresh at the start of next season. If there are any concrete offers yet? If this is the case, I am not aware."
Obviously, Manchester United can offer everything he is looking for, so I think an attempt to sign him would be a shrewd bit of business.
It is obvious, based on both last season and the season opener this year, that United lack a true warrior in the midfield. Fellaini could be the enforcer they truly need to help them regain the title.
Some fans may be against the signing simply because he plays for Everton, but many of these fans are also the people who want Javi Martinez despite probably having very limited knowledge of the player.
One of the reasons Fellaini probably doesn't get the credit he deserves is simply because he is so versatile.
He played just off the front man on Monday but was all over the place, and United could do a lot worse than sign someone who could dominate the engine room for years.
It's fair to say that Fellaini has always been a solid player but hasn't consistently provided fans with other performances like Monday's.
But he always shows up against the very best teams, and that is something you can't put a price on.
He was Everton's best player in the 4-4 game at Old Trafford last year, and he has also scored some very good goals against the likes of Manchester City.
Considering he cost £15 million, I'm really not sure how much Everton would ask for, but it's not as if they are in a position to turn down a big bid.
If Fellaini was available for the reported £20 million, that is a bargain buy when you consider the improvement you would see in him simply because he is playing with better players.
Sir Alex Ferguson has quite a few central midfielders but none in the mold of Fellaini.
Scholes will be gone soon, Anderson is yet to cement a place in the team even though he has had over five years to do so, Fletcher is sadly still not match-fit and Cleverley is still very inexperienced.
Carrick and Kagawa are there, too, but neither one of them fits the mold of a box-to-box midfielder who can affect all phases of the game.
By the way, Kagawa impressed me a lot and looks to have a great future if he is given the creative license to roam around the pitch looking for the ball like he deserves.
It remains to be seen if United will try to sign any more players, but now that they have added a striker and defender, it seems natural to add to the weakest part of their team.
Simply put, Fellaini is a big personality who is hard to miss and the type of player who leads by example and can inspire his teammates to put in even more effort.
If the management really wants to regain the league crown, they really should consider opening up their purse strings for the man of the moment (and of the last few years, if you watched him properly).