City went into the international break off the back of a 3-1 win over United at the Etihad Stadium to put 12 points between themselves and Jose Mourinho's side and continue their streak.
The pair meet again in March at Old Trafford, where Fellaini feels his side could be the ones to end City's stranglehold.
"They will lose," he told Sky Sports. "They are good, they have good players but it is possible to win against them.
"Ok, we lost to them but we have the reverse game at Old Trafford and we will see.
"I think they are very confident. They play good football. They play forward, they have a good dynamic so let's see in the future."
Arsenal are the only side in Premier League history to go an entire campaign without losing, as their Invincibles side scooped the title in the 2003-04 season.
Despite the lead which City have already opened up over United, who are languishing in eighth, Fellaini does not feel the reigning champions are out of sight as far as the title race in concerned.
"I don't think that [there is a big gap to make up]," he added. "I remember when I arrived in England there was a top four, now it is a top six so every year is more difficult."
The 31-year-old signed a new contract in the summer until 2020 and is often the man Mourinho turns to in a time of crisis.
Just six of the ex-Everton man's 13 appearances this season have come from the start, but Fellaini is happy to play any role.
Fellaini stated: "It is not about my role. Sometimes the manager uses me defensively or offensively and when I can help the team I give my best.
"The manager gives me a lot of responsibility so I have to keep going.
"He has given me a lot of confidence, he trusts me and has given me an important role in the team.
"It is the manager who decides. If he needs me for 5-10 minutes, or 90 minutes. He is the man to know that. He knows better than anyone who is going to start or not."