Kalvin Phillips saw red as Leeds United succumbed to a 4-2 defeat away to Nottingham Forest.
Jack Colback netted a brace for the hosts, who overcame a United fightback in the second half with further goals from Daryl Murphy and Ben Osborn.
Despite an early injury to Forest's Michael Hefele, it would be United who got off to a disastrous start.
With five minutes gone, Colback pounced upon Adam Forshaw's weak back pass to round Bailey Peacock-Farrell and slot the home side into the lead.
Forest's Claudio Yacob then threatened to extend the advantage when his header hit the outside of the post on 13 minutes.
There was then controversy two minutes thereafter. As Leeds's Jack Harrison bore down on goal, Jack Robinson appeared to force him to the ground. Despite strident appeals for a red card, Robinson only received a yellow.
United began to settle into the match. Ezgjan Alioski headed wide from Harrison's right wing cross. Kemar Roofe then stung the palms of Costel Pantilimon with a powerful shot after beating the offside trap. Joe Lolley then cleared another Alioski header off the line.
But as they were gathering momentum and beginning to ask serious questions of their hosts, Marcelo Bielsa's team suffered a critical blow to their chances on the stroke of half-time.
Kalvin Phillips's crude lunge on Adlene Guedioura left referee Darren England with no choice but to send him off.
Lolley, after another misplaced pass from Forshaw, then had a gilt-edged chance to deal another hammer blow to Leeds. But Peacock-Farrell made a good, low save to deny him.
At half-time, Bielsa introduced Jack Clarke in place of Harrison - and to devastating effect.
In the face of adversity, United rallied and Clarke soon pulled them level with a low finish from inside the box that squirmed under Pantilimon in the 52nd minute.
And they soon turned the match on its head twelve minutes later. Pontus Jansson headed Pablo Hernandez's wicked cross back across goal for Alioski to fire home from close range.
The goal was redolent of the team's recent fightbacks against Aston Villa and Blackburn Rovers. But this would prove a bridge too far - Forest were soon level on 69 minutes.
A deep corner found Robinson, who nodded the ball down to Colback, and he drilled a low shot into the bottom corner to restore parity.
And just three minutes later, they had the lead for a second time. Another corner found its way to an unmarked Murphy, whose glancing header eluded the despairing clutches of Peacock-Farrell.
Leeds's salvaging the match now appeared improbable, and any doubt of Forest's victory soon dissipated when substitute Osborn added a fourth goal five minutes later.
Murphy, perhaps offside, raced in behind and found Osborn with a judicious pass. He cut inside the hapless Forshaw and swept a sumptuous shot into the top corner to make the points safe.
United threatened a consolation on several occasions. Roofe burst through on goal only for Pantilimon to deny him with a strong, one-handed save. He then headed over the crossbar from mere yards out in added time.
Then came the final whistle. Rapturous applause from three-quarters of the City Ground greeted it. The win takes Forest to within three points of the play-offs.
For Leeds, this must go down as an acutely disappointing day at the office. There was a case to be made for Robinson taking an early bath, but Phillips's challenge was inexcusable and left his team in the lurch. For all their spirit, United's, chiefly defensive, individual errors cost them dearly. Forest punished them with all the ruthlessness of a Middle-Eastern despot.
There are some crumbs of comfort. Elsewhere, promotion rivals Norwich City, West Bromwich Albion and Derby County all dropped points. This defeat is, therefore, not as calamitous as it could have been. But it is patently clear that much work lies ahead if Leeds are to secure a return to the Premier League. An FA Cup tie away to Queens Park Rangers on Sunday awaits them before they take on Derby in a crunch match next Friday. Bielsa, and his depleted team, has much to ponder.
Nottm Forest (4-2-3-1): Pantilimon; Darikwa, Hefele (Guedioura 5’), Fox, Robinson; Yacob, Colback; Cash, Carvalho, Lolley (Osborn 65’); Murphy (Appiah 90+3’).
Unused substitutes: Steele, Yates, Dias, Janko.
Leeds United (4-1-4-1): Peacock-Farrell; Ayling, Jansson, Phillips, Douglas (Roberts 78’); Forshaw; Harrison (Clarke 46’), Hernandez, Klich, Alioski; Roofe.
Unused substitutes: Huffer, Halme, Shackleton, Baker, Davis.