Derby boss Frank Lampard says he will look back at the rulebook after being sent off as the Rams’ 10 men were beaten at Rotherham.
The game came to life after a goalless first half when Tom Lawrence was shown a straight red card for a reckless challenge on Richie Towell.
Ryan Manning put the Millers ahead soon after, firing in from the spot after Kyle Vassell was adjudged to have been brought down in the Derby penalty area.
The visitors struggled to create clear chances before Lampard was sent off for arguing with the fourth official.
The former England international said: “I didn’t get a word from the ref. I need to go back to the rulebook myself because I do not want to be leaving the game. I was upset with a handball. If those are the rules, managers will get sent off every game.
“I like communication with officials but there was none of that. It wasn’t a penalty. They got a soft one but we didn’t get one.
“Everything was going against us. It was one of those games. Once the melee started you sort of read the script.
“We didn’t need a reality check, this is the Championship. The beauty of the Championship is that there is a game in a few days.”
Lampard was angered by the awarding of the penalty and by the referee’s decision to only book Rotherham full-back Joe Mattock for a foul on Mason Bennett.
But he would have also been concerned by his side’s failure to break down a hard-working Millers side.
Marek Rodak saved spectacularly to keep out Tom Huddlestone’s fiercely hit first-half effort, while Semi Ajayi recovered to dispossess Bennett after he was played through on goal late on.
The Rams managed only four shots on target as they slipped to seventh in the table, while Rotherham climbed to 14th having made it back-to-back home league wins.
“It was frustrating,” added former Chelsea midfielder Lampard. “Rotherham played well and deserved to win the game.
“We did not move the ball quick enough and the pitch did not help. We lost the game. We got a lot of things wrong. We didn’t play very well.”
Rotherham manager Paul Warne said:
“The quality of player they’ve got and quality of bench they’ve got, it’s going to be a really tough ask of the lads.
“But I’ve got really good coaches, we set up in a really good way to try and frustrate them and to try and counter-attack. The lads grew in confidence and really fancied it.
“Overall, I thought we were deserving of the win and we were excellent.
“If we play that well at home and really battle and scrap the way the lads had to, I think we’ll be okay.”