Warnock insisted Hendrick should have been shown a red card instead of the yellow he received for a lunge on Leeds' Ryan Hall in the 35th-minute of Derby's 3-1 win.
Derby were already leading 1-0 thanks to a goal from Conor Sammon and Leeds went on to level through Paul Green before goals from Jake Buxton and Ben Davies sealed the win for Derby.
But Warnock insisted the course of the match could have swung in his side's favour had Derby been reduced to 10 men.
"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 today 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."