Sheffield United assistant boss Kevin Blackwell has quit his post at Bramall Lane to become head coach at Yorkshire rivals Leeds.
Blackwell (left) was Warnock's right-hand man
In joining Leeds, former Huddersfield and Plymouth coach Blackwell will end a 16-year working relationship with Neil Warnock which began back in 1986 at Scarborough.
"It is something I just did not envisage happening so quickly," said Blackwell.
"I woke up as assistant to Neil Warnock and now I've agreed to become head coach at Leeds. It's unbelievable.
Neil is stunned really
"I have agreed everything with them and now it's up to the clubs to sort out compensation."
Blackwell's decision leaves Blades boss Warnock with a huge void to fill.
"Neil is stunned really," added Blackwell. "There's no doubt about that - to lose the backbone of your coaching staff - it's a big blow to him."
Warnock had given Blackwell free rein on the training pitch in recent seasons and the 44-year-old has responded by getting the best out of a talented crop of youngsters at Bramall Lane.
"I would like to go on record and thank Neil for giving me the chance to express myself," said Blackwell.
"Neil is a very, very good manager and once he believes in you he lets you get on with it - he gave me that freedom on the training pitch.
"I would add that a coach is only as good as the players he's working with and we have been very, very lucky with the current youngsters at Bramall Lane."
Reid has spent the last few weeks mulling over the potential choices for his new backroom team after letting go assistant Eddie Gray and head coach Brian Kidd.
Blackwell initially worked under Warnock at Scarborough, joining the club in November 1986, proving pivotal to the Yorkshire side's promotion to the Football league that season.
He later joined Warnock at Notts County, and although he never played a first-team game for the Magpies, the Meadow Lane club were promoted from the old Third Division to the First via the play-offs in successive seasons.
Blackwell then followed Warnock to Huddersfield for two seasons from 1993-95, playing four games in the latter term which saw Town go up again via the play-offs into the First Division.
Warnock worked the play-off oracle again in 1995-96, this time at Plymouth where Blackwell played 20 matches in that particular promotion campaign.
The duo's 100% play-off record came to an end earlier this month when Sheffield United lost in the final to Wolves.