Northampton striker Marc Richards will be out for six to seven weeks after a scan showed he strained medial knee ligaments in training last Friday.
Richards has scored two goals in nine appearances this season
Cobblers boss Colin Calderwood said: "The good news is he does not need an operation, which we feared he might.
"Marc has been in a lot of pain since the incident, which was innocuous. He jumped for the ball and fell badly.
"It is a blow to Marc and we will miss him but his absence will give opportunities to other players."