Pep Guardiola’s side, who broke a Premier League record last season by winning the title with 100 points, have only dropped four points from their opening 12 league matches this term and have already firmly established themselves at the summit of the table.
Their latest victory came in a 3-1 win against United in the first Manchester derby of the season earlier this month.
Much was made of City’s domination against Jose Mourinho’s side on the day and the defeat left the Red Devils seven points adrift of fourth-placed Spurs.
Fellaini, though, is refusing to dwell on his side’s defeat, and when asked if the result proved the significant gulf in quality between the two sides, the Belgian told Sky Sports : "I don't think so. I remember when I arrived in England it was a top four, now it is a top six, so every year it is more difficult.
"Next season we will see how it changes and in two and three years I think every year it is more difficult, the intensity is higher, so that's why the Premier League is attractive.
"Manchester City are strong, but I think they will lose. Ok, they are very good and they have good players but it's possible to beat them. We lost, but we have the second game at Old Trafford and we will see."
Liverpool are the team hot on City’s heels this season and currently sit two points behind the champions in second.
However, Guardiola’s side look determined to further improve on their hugely impressive previous campaign and have only dropped points in the Premier League in away matches against Wolves and Liverpool so far.
Fellaini has admitted that there is much to admire about United’s local rivals, particularly their attacking style of play.
"They are very confident, all of them, they play good football, they play attacking football and they have a good dynamic,” the midfielder added.
City’s next league match will see them take on West Ham this Saturday, whilst United are set to host Roy Hodgson’s Crystal Palace on the same day.