The pressure on Derby manager Phillip Cocu intensified after his side were beaten 2-0 at home to Barnsley to leave them bottom of the Sky Bet Championship.
It was Derby's fifth home league defeat of the season and the second in four days to pile more misery on Cocu who is self-isolating after the club's chief executive tested positive for Covid-19.
With a takeover of the club pending, it was the last thing Cocu needed and his team's plight was summed up by a mistake from goalkeeper David Marshall which gifted Conor Chaplin Barnsley's opener.
They could have scored more goals before a fine finish by Victor Adeboyejo seven minutes from time sealed Barnsley's third win in four Championship games.
Derby had responded to Wednesday night's home defeat to QPR by bringing in Krystian Bielik for his first senior game since he damaged knee ligaments in January.
The Poland defender was one of three centre-backs but Derby were opened up in the third minute and it needed an excellent save by Marshall to keep out Dominik Frieser's low shot.
Derby had a chance two minutes later when Martyn Waghorn went past Jack Walton but his pass to Wayne Rooney was cut out by Michal Helik.
Walton rescued Barnsley twice in the space of a minute when he kept out a fierce drive from Tom Lawrence following Nathan Byrne's run in the 21st minute before beating away another effort from the Wales international.
Derby were building momentum until Marshall's attempted clearance went straight to Chaplin, whose drive beat the goalkeeper as he desperately tried to get back.
Barnsley almost scored again when Alex Mowatt fired just wide two minutes before the break but Derby went even closer through Jason Knight who was denied by a goal-line clearance.
Derby made an attacking change, bringing on Kamil Jozwiak at the start of the second half and the home side began to threaten with Waghorn curling a free-kick just over the crossbar before Lee Buchanan fired into the side netting.
Barnsley had a great chance to increase their lead in the 67th minute when Callum Styles was played in on the left but he fired wide and minutes later Chaplin wasted another opportunity with Derby's defence wide open.
The visitors were getting plenty of chances to put the game beyond Derby and they finally doubled their lead in the 83rd minute through a superb finish by Adeboyejo.
Barnsley broke quickly and Adeboyejo advanced to the edge of the box before curling a low shot past Marshall.
After that, there was no way back for Derby, who have now gone nine home games without a win in the Championship.
What the managers said...
Derby first-team coach Liam Rosenior: "I think the performance summed up the start of our season.
"The manager is a big man and will take responsibility but we all need to take responsibility too and I include the players in that - we need to look at ourselves in the mirror. Trust me, I go home tonight and I'm not sleeping because it hurts."
Barnsley's Valerien Ismael: "It was a big win for us, first of all for the attitude, the performance from the team. We wanted to be aggressive and have high intensity in the game.
"I told the guys before the game, 'force mistakes, punish mistakes', and it was a perfect game plan for us that's why I'm very happy about the result, the performance and I'm told the first win since 2009 against Derby - that's why it was the perfect afternoon."