Manchester City manager Manuel Pellegrini has admitted that the club are still unaware of what problem defender Matija Nastasic is carrying.
The 21-year-old has been out of action with a knee injury since the North-West outfit's defeat to Chelsea in February.
"It is very difficult because the doctor cannot find what happened with him," Pellegrini said in Friday's press conference.
"We made a lot of different things with him but he continues with pain so I don't know how many days he will continue with pain."
City will travel to Anfield to take on Liverpool in a potential title decider on Sunday.
