Bernardo Silva feels Manchester City dealt a damaging blow to Manchester United's hopes of winning the Premier League on Sunday.
City, the champions and league leaders, opened up a 12-point lead over their neighbours with a deserved 3-1 derby win at the Etihad Stadium.
It was United's fourth defeat in 12 games and left them eighth in the table.
City manager Pep Guardiola had said before the game that United could not be written off as challengers even if they lost, but Silva thinks they could have their work cut out.
The Portugal international said: "It won't be easy. Not only Man City - [they are] 10 points from Liverpool, a lot of points from the other ones as well.
"They need to be almost perfect and wait for the other ones to lose a lot of points. I think it won't be easy."
City show their superiority over United but it was not until the latter stages the points were secured.
City have dropped just four points this season and have scored 36 goals from 12 games, conceding just five. At this stage last year, however, they already had an eight-point lead at the summit. This time they are just two clear of Liverpool.
"We know it won't be easy this season," Silva said. "Liverpool are doing a fantastic Premier League as well.
"We want to try to and have a bigger gap during the months of December and January, where you have a lot of games. We will work very hard for that and try to improve on this."
Meanwhile, the FA has sought an explanation from Guardiola following his comments about referee Anthony Taylor prior to Sunday's derby.
It is understood the FA has contacted the Manchester City manager to request his observations and he has until 6pm on Thursday to respond.
Taylor's appointment for City's clash against United at the Etihad had caused some disquiet among home supporters.
Taylor hails from the Manchester suburb of Wythenshawe, which because of its proximity to Old Trafford was an issue for some City fans. Yet he is a declared Altrincham fan and the Premier League had no issues handing him the high-profile fixture.
Guardiola commented on this when the subject was raised at his pre-match press conference on Friday. The FA has previously warned managers for speaking about referees before games.
Guardiola said: "He is going to try to do the best job, like we try as managers and football players.
"Mr Taylor is going to try to make a good time. I don't think too much. The important thing is to focus on our game, of course knowing which opponent we are going to face, and that's all.
"Hopefully they can make good decisions for both sides and (it will) be a good game - all England, all parts of Europe are going to watch us, that is the most important pleasure.
"The referee doesn't want to make a mistake but he knows everybody around the world is going to watch him. He doesn't want to make mistakes, that is pretty sure.
"But if he is a fan of United or City? Anybody can be a fan of whoever he wants. No problem."
Taylor did not make any controversial decisions during the match.