Chesterfield will be without Craig Clay for around a month after he was diagnosed with a double hernia.
The League Two club expect the midfielder, 20, to miss four or five weeks with the injury.
Manager Paul Cook told the club's official website: "Last week against Gillingham we suspected for a while that he's not moving freely.
"Unfortunately, he's got the injury he's got. We can't do anything about it."
Spireites winger Tendayi Darikwa has been out for a month with the same problem.