Spireites midfielder Angel Martinez has been ruled out for six months after suffering a serious knee injury in training earlier this week.
The Spaniard has started the last three games for Chesterfield - and helped earn seven points - since signing for the club last month.
Rodger Wylde, the club's head of sports science and medicine, explained how the injury happened and the prognosis for Angel's rehabilitation.
He said: "Angel was training on Tuesday and he went of for a ball in the last minute of the session. He collided with a team mate and his left leg was forced into hyper-extension and he heard something snap.
"He came back to the ground and we assessed him and all the tests were positive for an anterior cruciate ligament (ACL) rupture. We arranged for him to see a specialist on Wednesday morning and an MRI scan confirmed what we thought.
"All the other ligaments are intact, and the prognosis now is for Angel to have three to four weeks for his knee to settle down, he'll do some strength work and some cycling to go through the full range of movement.
"Then he'll have a full reconstruction of the ACL and a return to playing is expected five months after the operation, all-in-all it'll be six months from now."
Manager Dean Saunders reacted to the setback, commending Angel on the impact he has already made in his short time with the squad.
"It is very disappointing for the boy," said the manager. "He's made a big impact since he arrived and made a big difference to Sam Morsy and Jay O'Shea, helping us keep back-to-back clean sheets.
"We are all disappointed to lose Angel, but that's football and we have to accept it and carry on."