A club statement says that he moves for "£750,000 plus significant incentive add-ons," which are reported to take the fee up to a total of £1million depending on his appearances for the club and if they achieve promotion. City will also receive further money through a sell-on clause should Fagan leave Derby for a fee. No such clause was agreed when City bought him from Colchester almost 2 years ago, so the Tigers receive the full amount of his sale.
Fagan will also have to sit out the game between Derby and City at Pride Park on 10th February.
"One or two experienced players" and an addition to Phil Brown's coaching staff will be coming to Hull this month.