Manchester - Manchester City captain Vincent Kompany could miss the rest of the season, manager Manuel Pellegrini said on Friday.
Kompany failed to return for the second half after suffering a groin injury in the struggling champions' 4-2 derby loss to Manchester United at Old Trafford.
Now the Belgium centre-back will miss the Premier League match against West Ham at Eastlands on Sunday and there is no guarantee as to when he will return.
"He is injured. It is very difficult to know the way you improve if you are injured," Pellegrini told a news conference on Friday.
"He has a muscle injury, we don't know how many weeks.
"We always know the (approximate) amount of weeks he needs to recover but we don't know if he will be fit from now until the end of the season."
For Belgium skipper Kompany, 29, this has already been a difficult season, with the central defender widely criticised for failing to reproduce the form that helped City win the title last season.
This campaign has also seen Kompany suffer the latest in a long line of leg muscle problems, with the City cornerstone missing nine games -- mostly in December -- with calf and hamstring injuries.
Last weekend's loss all but scuppered City's hopes of a successful title defence, putting them 12 points behind leaders Chelsea, having played a game more than the table-toppers.
Indeed their sixth defeat in eight games in all competitions left City in fourth place and knowing they cannot afford to slip any lower if they are to secure a place in next season's Champions League ahead of the likes of Southampton or Liverpool.