Last season's champions and runners-up meet at the Etihad Stadium, hoping midweek Champions League success can ignite their domestic campaigns after some uninspired performances from both in their first four games, each taking just seven points.
While Wayne Rooney's return to form in Tuesday's 4-2 win over Bayer Leverkusen stole the headlines, Fellaini's physical presence and ability to hold the ball in his first start since signing just before the transfer deadline added a new dimension.
It was no coincidence that a side who had not scored in two of their last three league games and only netted from set pieces in the other were suddenly able to put together more meaningful charges forward with the big Belgian on the pitch.
Similarly, City, who had failed to repeat the scintillating showing of their season-opening demolition of Newcastle United, rediscovered some attacking flair in a stylish 3-0 win at Viktoria Plzen on Tuesday.
"It will give confidence for all the players," City manager Manuel Pellegrini told reporters.
"We have the derby next Sunday and always winning away in the Champions League, scoring three goals, having at least three or four more chances to score and a clean sheet - I think it will give all the players a lot of confidence."
City midfielder Yaya Toure, who like Fellaini offers a towering presence, creative instinct and ability to read a game, scored a sublime goal at Plzen and it could well be the battle between these two powerful players that proves key on Sunday.
It will be the first Manchester derby for both clubs' managers and they will be aware that City hold a slight advantage in recent years having taken 10 points to United's seven in league encounters in the past three seasons.
The return of captain Vincent Kompany, who made a successful comeback from a groin injury on Tuesday, will offer the hosts a further boost ahead of what he describes as a "special game."
"It is becoming one of the most sought-after fixtures in the world and it is always great to be involved in such games. I never take them for granted," the Belgian defender said.
This fixture two seasons ago, which City won 1-0, proved decisive as they went on to secure the Premier League crown on goal difference from their neighbours.
It is far too early in the season to be talking of the title race but it is nevertheless a crunch game with both sides keen to establish the edge on their opponents.
Early season pacesetters Liverpool, who are unbeaten with 10 points from four matches, host Southampton on Saturday but have suffered a blow with midfielder Philippe Coutinho ruled out until the end of next month with a shoulder injury.
They are, however, full of confidence over the form of striker Daniel Sturridge. He has netted in all their league matches this season and will be keen to keep up his scoring escapades before the return of Uruguay striker Luis Suarez from a 10-match ban, which comes to an end after this game.
Later on Saturday, Chelsea host Fulham in a west London derby eager to make amends after suffering their first league defeat of the season at Everton last weekend and losing at home to Basel in the Champions League.
North London rivals Arsenal and Tottenham Hotspur, who both have nine points, are in action on Sunday, with Arsene Wenger hoping for more magic from record signing Mesut Ozil when they host Stoke and Andre Villas-Boas taking his team to Cardiff.
Ozil enjoyed an exciting Arsenal debut, taking only 11 minutes to create a goal against Sunderland, and team mate Aaron Ramsey, whose rich scoring vein continued in the 2-1 Champions League win at Marseille, said it was just the beginning.
"You saw what Mesut is all about with those lovely, telling through balls to set people up one on one with the keeper," local media quoted him as saying. "That's why he is top of the assist chart around the whole of Europe.
"He is an unbelievable talent and hopefully that is just the start of many things to come from him.
Two of the early season strugglers, West Bromwich Albion (two points) and Sunderland (one), meet at the Hawthorns on Saturday, while fellow slow starters Crystal Palace entertain Swansea City on Sunday.