City won 3-1 in the Manchester Derby before the international break, leaving Man United in 8th position in the Premier League as Jose Mourinho faces repeated questions over his future at the club.
However, midfielder Fellaini believes the gap between the two sides is not as big as the league table suggests.
"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," the Belgian said to 'Sky Sports'.
"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."