Hendrick received a yellow card instead for a 35th-minute lunge on Ryan Hall, much to Warnock's anger.
"How Hendrick did not get a red card I will never, ever know," he said. "It's a horrendous challenge, it's a career-threatening challenge and to get a yellow card for that, I do not understand the game.
"They're even saying Ryan Hall got up quickly, as if that's a crime. What do we tell them to do, stay down and roll around? It was bad at the time but it's even worse when you watch it again.
"I thought we were poor for the first half hour but we withstood that and I didn't think they could improve but I thought we could be 100 per cent better in the second half and I thought we were.
"The second goal was always going to be the winner and we couldn't quite manage it. You need the rub of the green at times but it would have been entirely different with 10 men."
There was little to warm the fans on a cold afternoon until Derby took a quick free-kick inside the Leeds half in the 15th minute and Hughes' astute pass sent Conor Sammon in to beat Paddy Kenny with a low shot from 12 yards.
Leeds had been second best but they scored just before the interval when Lee Peltier crossed from the left and Luciano Becchio headed on for Paul Green to turn the ball in.
Will Hughes almost restored Derby's lead in the 59th minute when he beat two defenders and fired in a shot which Kenny turned away. But the keeper was beaten again in the 66th minute when Leeds failed to clear a corner and Richard Keogh drove in a low shot which went in off Jake Buxton in a crowded six-yard box.
Ben Davies then came off the bench to settle the contest in the last minute of stoppage time, when he bent an unstoppable shot from just inside the area to keep Derby's amazing run against Leeds going.