Steve McClaren was on Sunday night sacked by Derby for the second time in less than two years after a five-month second spell.
The 55-year-old former England manager returned to Derby last October, following his May 2015 dismissal, but a wretched run of results saw him axed.
McClaren's five months in charge ended in the wake of Friday's 3-0 loss at automatic promotion-chasing Brighton.
Chairman Mel Morris said "the heartbeat, commitment, focus and passion has been lacking something fundamental".
The Rams have won once in nine Sky Bet Championship matches since the turn of the year - against Barnsley on 4 March - and are 10 points adrift of a play-off place with nine games remaining.
Bookmakers made former Derby defender Gary Rowett, previously boss of Burton and Birmingham, an early favourite for the job.
Ex-Newcastle boss Alan Pardew and Roy Hodgson, another former England manager, were also in the betting, alongside Mark Warburton, Claudio Ranieri and Garry Monk.
Morris said: "Steve is a great coach, we have very talented players and first-class facilities but the heartbeat, commitment, focus and passion has been lacking something fundamental.
"The Brighton game on Friday night was so far from what we expect to see from those wearing a Derby County shirt."
The club statement pointed to the miserable run of form.
"Since the turn of the year we have experienced a significant, unexpected and persistent decline in results, team unity and morale," it said.
"Our form over the last 10 games would place us above the relegation group by goal difference alone.
"The club expects to make an announcement in relation to Steve's successor in the next few days."
Morris insisted Derby would prioritise "getting things right" in appointing the next boss.
Morris added: "We need a manager who shares our values and who is prepared to develop the team, staff and processes that will set us on course for a sustainable place in the Premier League.
"We are less concerned about timescales than we are about getting things right. Of course we would like to achieve this sooner rather than later.
"That manager will of course be given the full support and backing of myself, the board, our executive team and staff to shape and develop the team around those who truly and demonstrably want to be part of Derby County."
