There are fears the Serbia centre-back could miss the start of the season after being carried off on a stretcher during the Asia Trophy clash against Sunderland in Hong Kong at the weekend.
The 20-year-old, who impressed after joining the club from Fiorentina last summer, has not travelled with the squad for this week's Audi Cup tournament in Munich.
Manager Manuel Pellegrini, speaking at a press conference in Germany, said: "We will have to wait to find out how serious the injury to Nastasic is.
"At this moment, we don't know whether he needs an operation or if there is another solution but I have a lot of players that I can rely on.
"There are enough players who can come in to replace any that are missing."
With Kolo Toure having left City for Liverpool, Nastasic's injury could allow Joleon Lescott the opportunity to reform his successful defensive partnership from the 2012 title-winning season with Vincent Kompany.
City have otherwise taken a strong squad to Germany as they face AC Milan on Wednesday and another game against either hosts Bayern Munich or Sao Paulo on Thursday.
Pellegrini said: "The whole squad is here except for Matija.
"I don't know who will play against Milan yet. It's more important we see who is fit and who is ready to play."
- Sports & Recreation
- Manuel Pellegrini