The striker, on-loan from Southampton, raised his tally to eight goals in 18 games and was at the heart of Forest's Boxing Day victory.
Sharp's first goal, a penalty just before half-time that equalled Forest's lead, was, according to Warnock, the start of his side's downfall.
"I've known Billy a long time and if you are up against him in the area he's going to be clever," he said.
"Tom (Lees) has just breathed on him but Billy has made sure there is contact and gone down. In that situation you have got to stand a yard off him.
"It's a penalty because 20 out of 20 referees would give it. Billy used to get penalties for me like that at Sheffield United so I can't complain. Tom has just got to be more professional.
"It was really disappointing because at that stage it was the only way I could see Forest scoring.
"The equaliser gave Forest the lift they needed and we were guilty of being very amateurish and naive in the second half.
"If you concede three goals in seven minutes at any level you can't expect to win football matches, but at the end of the game I'm still disappointed that we have passed by another chance to win a game."
Forest boss Sean O'Driscoll strongly defended Sharp, he said: "We all know there are players throughout football who are likely to go down easily for penalties but Billy is not one of them.
"I don't think anyone in their right mind would say that Billy Sharp is a diver.
"What he does do is score goals. He's made his name doing that and if you give him chances he puts them away.
"He had a bit of pressure with the penalty in front of the Leeds fans because it was an important moment in the match. But he showed his quality when he was one on one against the keeper to score his second.
"Like one or two of the other lads he was a bit under the weather before the game but the two goals will not do him any harm."