Midfielder Mark Randall has left Chesterfield having reached the end of his contract with the League Two club.
The former Arsenal trainee has not been offered a new deal and leaves the Spireites after two years at the Proact Stadium.
"We're still looking for a central midfielder but it will be a strong, aggressive, holding type of midfielder," Blues boss Paul Cook told the club'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 the Gunners. 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.