Goalkeeper Tommy Lee is facing six to eight weeks out with ligament damage in his shoulder.
The 26-year-old landed awkwardly after coming to claim a ball into the box in the closing stages of Chesterfield's 2-1 victory over Oldham on Tuesday night.
The positive news is that, after being sent to a specialist, the club has been told that surgery is not required.
Reacting to the news, caretaker-manager Tommy Wright said: "Obviously it's a blow to lose Tommy but the positive news is that he doesn't need surgery; it could have been a lot worse.
"We've sent him to a top specialist, somebody who looks after the British Lions rugby team, so he's in good hands.
"He's a hard worker and the specialist is confident that he can make a full recovery within six to eight weeks."
Lee was ruled out for a similar period in September 2011.