The Chilean wants to avoid complacency against the newly promoted side and adds that he expects no more departures this summer after Scott Sinclair and Denis Suarez's exits
Last year's Premier League runners-up were in superb form in their opener against Newcastle on Monday, easing to a 4-0 win thanks to goals from David Silva, Sergio Aguero, Yaya Toure and Samir Nasri.
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"We have a very difficult game on Sunday because Cardiff are playing their first home Premier League game," the former Malaga coach told reporters.
"We will not underestimate them because they are new to the division. They will be very tough opponents on their own pitch. We know we have to play our best to take the three points but we will be focused and prepared for the challenge."
Pellegrini added that it is unlikely that any more players will depart the Etihad Stadium before the end of the transfer window after Scott Sinclair moved to West Brom on a season-long loan deal and Denis Suarez signed for Barcelona.
"I'm not certain but I doubt there will be any more players leaving," he said.