He missed much of last season after twice breaking the same metatarsal bone in his left foot.
Mowbray said: "The supporters are well aware that Curtis wanted to leave the club and play in the Premiership.
"I want to thank him for his efforts and total commitment to the cause last season.
"However, since our defeat at Wembley (in last season's play-off final), he clearly felt his future lay elsewhere. This has resulted in Curtis not wanting to play for the team this season.
"Although he's a player of undoubted quality, we only want players here who are fully committed to the cause, which is what I've stressed all summer.
"Curtis hasn't contributed during the first five games and, with there being plenty of competition for places in central defence, I think this deal suits all parties."