Midfielder Mark Randall has left Chesterfield having reached the end of his contract.
The former Arsenal trainee has not been offered a new deal and leaves the Spireites after two years with the club.
"We're still looking for a central midfielder but it will be a strong, aggressive, holding type of midfielder," Blues boss Paul Cook told Chesterfield's official website.
"We thank Mark for everything he's done and wish him every success for the future."
Randall, 23, joined the Spireites in the summer of 2011 after being released by Arsenal. During his stay in Derbyshire he made 51 appearances and scored four goals.
Cook has recently strengthened his central midfield options with the signings of Jimmy Ryan and Sam Morsy.