Pellegrini revealed that Silva has suffered a calf injury and will likely be on the sidelines for three to four weeks, which could rule him out of the next four Premier League games.
The Chilean did say that centre-back Vincent Kompany was recovering well and could be fit to face Tottenham Hotspur after the international break.
"Silva has a problem with his calf, I think it'll be three or four weeks," Pellegrini told reporters.
"The first diagnosis [for Kompany] was five weeks and that was three weeks ago. We hope he continues the same way."
City travel to Sunderland on Sunday afternoon.