The 20-year-old impressed in his first season in the Premier League but his second term started badly when he was stretchered off in the Barclays Asia Trophy final win over Sunderland with an ankle injury.
Nastasic did not travel with the City squad to Germany, where they are playing in the Audi Cup tournament, and Pellegrini was unable to provide much of an update on his condition.
"We will have to wait to find out how serious the injury to Nastasic is," Pellegrini said.
"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.
"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."