Ahmed Elmohamady snatched a dramatic late winner to fire Hull up to second in the npower Championship, after fellow Egyptian Gedo had made an instant impact on his home debut.
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Steve Bruce's men delivered a dismal first-half display against play-off chasing Derby, prompting the introduction of Gedo, a recent loan signing from Al Ahly, and Nick Proschwitz at the break.
Gedo, who also came off the bench against Brighton at the weekend, headed Hull in front within 90 seconds of his half-time arrival.
However, after both sides had struck the woodwork, Jamie Ward looked to have rescued a point for the Rams when he capitalised on an error from City goalkeeper David Stockdale to level with six minutes remaining.
There was still time for Elmohamady to have the final say with a downward header from Robbie Brady's corner, and Derby finished with 10 men following Gareth Roberts' 89th-minute dismissal for a late tackle on Proschwitz.
Corry Evans replaced the injured Alex Bruce in Hull's line-up, while Derby were also forced into a solitary change, with Kieran Freeman coming in for Jake Buxton and John Brayford switching from right-back to centre-half as a result.
Hull made a desperately slow start, enabling the visitors to enjoy the better of a forgettable opening period.
Conor Sammon spurned a good chance to fire Derby ahead in the eighth minute, poking wide from inside the area following good interplay with Ward and Paul Coutts.
Ward was then denied a clear run on goal by Jack Hobbs' last-ditch tackle and Craig Bryson fired over after cutting in from the left.
Having frustrated their fans with some sloppy passing and generally ponderous play, Hull finally threatened as the half hour approached and Robert Koren should have done better when missing the target with a header from Elmohamady's cross.
Derby continued to look sharper and Jeff Hendrick saw an effort deflected behind for a corner after another neat move.
Yet City almost took the lead on the stroke of half-time when Stephen Quinn's glancing header slid past Adam Legzdins' right-hand post.
Hull did not have to wait long for a goal after the interval as Gedo, brought on with Proschwitz in place of Koren and Jay Simpson, met Brady's pinpoint centre with a firm nod back across Legzdins.
The substitute soon went close again, volleying over from Elmohamady's cross as the game belatedly sparked into life.
Hull's David Meyler then saw a shot deflect on to the upright, before Sammon struck the underside of the bar for Derby, having been played in by Ward's clever chipped pass.
A goalbound effort from Ward almost ricocheted in off Sammon moments later, but the former's luck changed when Stockdale failed to gather Bryson's left-wing cross, leaving a simple chance that was gratefully taken.
Hull's disappointment was short-lived as Elmohamady converted Brady's 87th-minute corner.
Proschwitz then missed an open goal in injury-time following Roberts' straight red card.