McGreal suffers new injury blow
The 30-year-old defender was stretchered off in Tuesday night's reserve clash with West Ham.
The swelling is so bad on the knee that McGreal will have to wait a week before undergoing tests to find out the full extent of the injury.
Town boss Joe Royle admitted it was a blow to lose the former Tranmere ace.
"I feel terrible for John, he is a thoroughly nice man who doesn't deserve this," said Royle.
"And it has hurt us not having him. He was in outstanding form until he got the original injury at Brighton and since then we have not had many clean sheets. We would certainly be points better off if we had had him fit all season."
McGreal was distraught at suffering the new injury blow.
"I'm gutted. I can't believe my luck," McGreal told the club's official website.
"I've been told it could be my medial ligament, we need to wait for seven to 10 days to let the swelling down before I can have a scan to reveal the extent of the damage."