Nemanja Matic has warned Manchester United they have to be ready to face a backlash after their rampant start to the Premier League season.
The £40million midfielder says the road ahead is going to get tougher for Jose Mourinho's pacesetters.
The Reds are joint top of the table with City and have been blowing away opponents with 21 goals scored in the league on top of seven in the Champions League and four in the League Cup.
But Matic, who won the Premier League title twice with Chelsea in 2015 and 2017, suggests the opposition will get clued up on facing United.
"We cannot be relaxed for the future because we have to win more, focus because the teams now know how we play, the teams knows our formation, our players and, of course, in the future those will want to show they can play against Manchester United," said the midfielder.
"We will be focused, we will be ready for that. It is going to be hard but that's why we are here.
"As you know, we win almost every game, we only draw at Stoke - in the Premier League it is very hard to do this," he added. "This can only be done by big teams, so we showed that we are a big team and we have to continue like this." Meanwhile, Zlatan Ibrahimovic has been warned against making an early comeback for United by surgeon Dr Freddie Fu, who operated on his knee in May.
United boss Jose Mourinho has been more cautious and suggested the 35-year-old will not be ready until January.
"I have no worry about his injury as it always takes time to fully recover," Dr Fu, chairman of the Department of Orthopaedic Surgery at the University of Pittsburgh School of Medicine, said.
"As a doctor, of course I want to see him return as slow as possible, only when he is fully fit for the game. There have been many cases of a player returning prematurely with bad consequences."