All Whites captain Winston Reid will miss the first two months of the upcoming English Premier League season after having surgery.
The West Ham United defender returned early from a pre-season training camp in Switzerland to undergo knee surgery.
West Ham's striker Andy Carroll will also be out for the first two months after undergoing ankle surgery
"This is a big blow for them and us but we hope to see them back as soon as possible," said boss Manuel Pellegrini.
"It does mean that we have lost two players in key areas, though, and we'll continue to work hard with the owners to make further additions to strengthen the playing squad, beyond the five new signings already confirmed," he added.
Reid has made 222 appearances for the club since joining in 2010, while Carroll - who last played for England in 2012 - scored three goals in 18 appearances last season.