Leeds took control of the two-legged tie with a 1-0 win at Pride Park but Lampard hit out at referee Craig Pawson over a 78th-minute incident which looked to have earned Derby a spot-kick.
Pawson pointed for a penalty after Jack Harrison and Jayden Bogle collided in United’s box as Harrison attempted to clear a through-ball from Harry Wilson.
Assistant Eddie Smart, however, called Pawson over and advised the South Yorkshire official to reverse the decision and give Leeds a free-kick instead.
Derby protested angrily and Lampard was critical of Pawson for delegating authority to his assistant, despite admitting that the collision between Harrison and Bogle involved “minimal contact”.
“The penalty is one which could go either way,” he said. “I think it was (a penalty) because Bogle gets his leg across his man.
“Contact was minimal but the astounding thing is that when the ref gives it, he’s the boss on the pitch but he consults his linesman. You think there’d be some big information which the referee hasn’t seen and I don’t think there is. It’s like VAR without the computer system.
“I’m not sure it’s a penalty but I’ve never seen a decision overturned unless there’s something clear which hasn’t been seen, and that’s not the case. If we all looked at it in this room, some would say yes and some would say no, I get that. I’m just surprised a referee in a game of this magnitude has decided to play second fiddle to his assistant.”
Speaking to Sky Sports after the game, Harrison said: "I was a bit surprised.
"I was only going for the ball, I had no intention to take him down or anything like that. We try and play a fair game and what happens happens.
"I felt he came from behind me but it’s all about perspective. That’s just my point of view, it could be different from everybody else.”
Bielsa said he had been denied a clear view of the coming-together.
"We were very far away from the action,” Bielsa said. “We had many players in front of us. Honestly, I couldn't give you my opinion.”