Northampton Town striker Marc Richards will be out of action for around three months and may need an operation.
Richards was stretchered off with a knee injury last Friday against Chester and had a scan on Monday.
Northampton boss Calderwood told the club website: "It's a big blow for us and for Marc.
"The scans suggest that he has a ruptured a medial ligament and he may require an operation. He is likely to be out of action for three months."