Liverpool are only behind City on goal difference, while Chelsea and Arsenal are clear of Spurs, but United are eighth and already nine points off the pace after winning just two of their last five top-flight matches.
And Silva omitted the Red Devils when discussing the biggest obstacles to City retaining their crown.
“I think it will be harder because last season nobody was expecting us to win as many games and to reach that level of points [100], so this season the teams know they have to do better to beat us.
“Of course, it will be very hard, but we will work every game to get the three points and at the end of it try and celebrate winning one more Premier League.