The attendance of 39,223 at Etihad Stadium was the third highest in the round, behind West Ham and Newcastle.
"Today was not full - I don't know why," the City manager said.
He added he hoped there would be "more people" and "more support" at Wednesday's Carabao Cup semi-final second leg against Manchester United.
"It is a chance to come back to Wembley for three years in a row," said the Spaniard, whose side have won the Carabao Cup for the past two seasons and lead United 3-1 after the first leg.
"Hopefully our fans can come, more people than today.
"Hopefully they can support us more and make an intelligent game to reach the final."