City have won 10 of their opening 12 league games, boasting a two-point lead atop the table.
In their last fixture, Pep Guardiola’s men outplayed United in the Manchester derby to run out 3-1 victors.
Fellaini played in that game but could not fill the boots of the injured Paul Pogba, who scored two goals in United’s comeback win at the Etihad in April.
In stark contrast to their local rivals, United are languishing at eighth in the league, already seven points adrift of top four and 12 points off City.
The loss before the international break showed the class between both sides in their current state, but Fellaini believes not much should be read into it.
“I don’t think so,” Fellaini said, when asked by Sky Sports whether it was a fair reflection of both teams’ quality.
“I remember when I arrived in England [in 2008] it was a top four, now it is a top six, so every year it is more difficult.”
Fellaini seems to relish the heightened competition, in fact, saying: “Next season we will see how it changes 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.”
City only lost two league games last year and don’t look like suffering their first defeat anytime soon, but Fellaini does not think they will become only the third side in top-flight history to go a season unbeaten.
“Manchester City are strong, but I think they will lose,” the Belgian added.
“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.”
Pushed more for a positive comment about Guardiola’s side, Fellaini admitted: “They are very confident, all of them.
“They play good football, they play attacking football and they have a good dynamic.”