The Spanish playmaker looked out of sorts against Southampton in City's opening game, and was then dropped for the match against Liverpool.
However, Mancini thinks that fans should be patient.
Silva played over 55 games last season and was a key part of the Spain side that went all the way at Euro 2012. On top of that he played an international friendly in the Americas just before the season started.
The Italian says that it could take a while before all his international players are back in top form.
"We don't have all the players in good form," he told the Manchester Evening News.
"The international players started pre-season on July 25. They are not in good form and that is one of the reasons.
"David started his pre-season late and for one week went to Puerto Rico with Spain. He needs training and maybe to play, but not for 90 minutes."
City's next Premier League game is againts Queens Park Rangers on Saturday evening.