A new signing at the club for the 2016/17 season, Turnbull joined on a three-year deal from Southampton for an undisclosed fee.
The former England Under-20 centre-half didn't make a senior appearance for the Saints but impressed whilst out on loan at Swindon Town, the club he started his youth career at.
Turnbull has spent the last two seasons at The Robins, making a total of 86 League One appearances. He scored his first two goals for the Sky Blues against West Ham United PL2 in the EFL Trophy victory.
Jordan made 45 appearances for the Sky Blues in his maiden season, but was sent out on loan to Partick Thistle at the start of the 2017/18 campaign.