A dramatic first half at Stamford Bridge saw five goals scored, with Chelsea leading 3-2 at half time.
The hosts took the lead when Derby defender Fikayo Tomori, on loan from the Blues, turned into his own net.
Jack Marriott equalised for the Rams before it was the turn of Richard Keogh to score an own goal.
Martyn Waghorn once again drew Derby level before the moment of controversy.
Chelsea right-back Davide Zappacosta appeared to push Derby winger Tom Lawrence to the ground.
The assistant, who was standing right in front of the incident, did not deem it to be a foul.
From the resulting throw in Chelsea scored through Cesc Fabregas.
And former Liverpool midfielder Redknapp was not impressed with the decision at half time.
“It’s a blatant foul,” he said.
“It’s a difficult one for Jon Moss because he needs his assistant to help him out there. It’s a definite foul.
“But after that it’s a good piece of play by Cesc Fabregas, that pass there is a typical Cesc Fabregas pass.
“He doesn’t just rest, he goes and wins the second ball. This is a clever piece of play, it’s a good finish.
“It’s 100 per cent a foul, I can understand why the Derby players are so upset with that one. He’s a yard away from it.”
Chelsea hero Hasselbaink agreed and could not believe a foul was not given.
“The linesman has to (see it), what are you doing?,” the Dutchman said.
“You can see there’s a proper push, the linesman can just put his flag up and it’s a free kick and nobody else would think anything else of it.”