Wayne Rooney's first match in permanent charge of Derby ended with a 1-0 defeat at home to Championship relegation rivals Rotherham.
The former England captain was officially confirmed as the Rams' manager on Friday, having held the post on an interim basis since the departure of Phillip Cocu in November.
But his reign got off to an inauspicious start as Jamie Lindsay's late strike sent County second bottom of the table.
Rotherham moved ahead of their opponents on goal difference and they also have two games in hand on the teams around them.
The standout news from Rooney's team selection was his decision to hand the captain's armband to 19-year-old Jason Knight.
Knight's midfield colleague Graeme Shinnie registered the game's first shot on target when he hit a rasping drive which visiting goalkeeper Jamal Blackman was more than equal to.
Rotherham then broke quickly, with Wes Harding picking out Michael Smith inside the area, but he could only plant an effort wide.
Lee Buchanan drilled an effort towards Blackman's right-hand post after 26 minutes, but the Chelsea loanee got down quickly to turn it behind for a corner.
Rotherham thought they had taken the lead 10 minutes before the break. Captain Richard Wood applied a fine finish inside the area but, after a brief delay, Wood was ruled offside and the goal was chalked off.
The second half was an even more cagey affair, with chances at a premium.
Martyn Waghorn fluffed a rare opportunity when well-placed after 70 minutes, firing wide of goal from just a couple of yards out.
The game livened up as it headed into the latter stages and Shinnie again tested Blackman from close range before Waghorn's looping header almost caught the keeper off-guard.
But, after soaking up pressure, the Millers snatched victory against the run of play in the 86th minute.
Shortly after having a goalbound effort hacked away by a back-tracking defender, Lindsay knocked home the winner from a few yards after Daniel Barlaser's free-kick had crashed against a post.
What the managers said...
Derby's Wayne Rooney: "Today Rotherham outfought us. Our quality on the ball and our concentration levels weren't where they should be. That's the disappointing thing and, if I'm being honest, I don't think we deserved anything from this game.
"Obviously, the preparations over the last few weeks have been hampered because of Covid but that's not an excuse for today's result. The lads looked good and sharp in training the last few days. We knew Rotherham press well and would be a tough game, but we didn't seem to have that quality on the ball.
"We seemed to lack edge and let ourselves down when in possession and under pressure. We know we can play better than we did today, but I don't think we even deserved a point today. We gave silly free-kicks away in dangerous areas and that's what Rotherham were looking for as it's a big part of their game. But those free-kicks allowed them to send defenders up and cause issues. You understand players making mistakes. But hopefully it's a one-off and we can respond properly."
Rotherham's Paul Warne: "I'm really pleased and I thought we played really well. We put some dangerous balls in in the first half but it didn't quite drop for us. I expected Derby to come out stronger in the second half and raise their game and they did have a little spell.
"It looked as though it was going to fizz out into a 0-0 draw, but we were determined to come here and get a win. They changed their system a couple of times and fortunately for us Jamie had two bites at the cherry. He should have scored the first, but thankfully we managed to get the goal. I'm really proud of the lads."