The two rivals occupy first and second spot in the Premier League table, with Tottenham, five points adrift of City, their nearest competition, while reigning champions Chelsea are nine points worse off in fifth place. City and United haven't locked horns this season yet, but the three Belgium mates, Romelu Lukaku, Marouane Fellaini and Kevin De Bruyne have shared the dressing room while playing for their national side.
Kevin De Bruyne believes the Premier League title race is already between Manchester City and United, but warned that could change quickly.
"For the moment I think so, but everybody's still there", De Bruyne told ESPN when asked if the Manchester clubs are favourites.
"Now, not only do we swap from a Bank Holiday game to an evening away game on 27 December, and then an away game at noon on New Year's Eve - to the most hard London game you can get to - but we also lose our only home game on New Year's Day". De Bruyne believes City is producing the types of performances its Spaniard boss wants. "We have to maintain this hopefully".
Pep Guardiola's side also made strong start last campaign when they won their first 10 matches under the former Barcelona coach.
"For the moment I think so, but everybody's still there".
Under normal circumstances this would seem a fairly straight forward home win for the Blues - but they do not seem like a team with any kind of fluency or harmony now, and last weeks loss to Crystal Palace made it two league losses on the bounce for Chelsea and their third of the season already.
"Especially at the moment, we are playing maybe a little bit better, but we're just building on what we did past year and, for the moment, everything is going well". "I think a year ago we played good football but, especially in the big games, we didn't win them".
"I think past year we played good football but especially in the big games we didn't win them", De Bruyne added.