Ges Steinbergs, the club doctor, says: "He has a tear which means it is a month before he will be back playing and he will be assessed through that time.
"Obviously there is a break after the Birmingham game so at least he won't miss as many games as he would otherwise have done and he will make a full recovery."
Fredi missed much of last season with a groin problem and was on a programme of specialist exercises as well as regular training, but an unlucky slip early on at Stamford Bridge means another spell on the sidelines.
As for the gruelling 120 minutes plus at Chesterfield on Tuesday evening, no major injuries are reported as a result.
"There are the usual knocks but nothing too out of sorts," says Ges.
As far as Don Hutchison is concerned, his progress is continuing apace, but despite reports no date has been set for a return to action.
"That would be counterproductive," says Ges, "as we don't want him overdoing it and picking up other injuries.
"But he is doing lots of running and progressing well."
Ian Pearce, who served a one match ban on Tuesday following his sending off at Spurs, returns to the squad for the visit of Birmingham City at the weekend.