Referee Michael Oliver missed the incident and a VAR review ruled against a red card for Lo Celso – but technology coordinators at Stockley Park later admitted their error and that the Spurs star should have been dismissed.
Olivier Giroud and Marcos Alonso struck as Chelsea cemented fourth place, but Blues manager Lampard was left fuming at Lo Celso avoiding a dismissal.
"Everybody knew they made a mistake," said Lampard.
"We have got the monitor on the side of the pitch and can view it.
"I was just waiting for the red card to be shown, not with pleasure, but that's a tackle that endangers a player.
"It's just not good enough, there's no more to say. Saying afterwards they made a mistake is not good enough. They had minutes, but they still made a mistake.
"It's another huge question mark on VAR.
"I want it to be good, I want it to work. VAR was brought in to remove mistakes and it was so wrong today. So it's more question marks.
"This situation didn't need the pitchside monitor. It's a clear decision, Stockley Park, red card, game goes on."
Smart strikes from Giroud and Alonso ensured Chelsea wound up avoiding losing out to VAR again, while Antonio Rudiger was unlucky to put through his own net late on.
Manchester United captain Harry Maguire somehow avoided a red card as Chelsea lost out 2-0 at Stamford Bridge on Monday night.
The Blues also saw two goals chalked off by VAR against the Red Devils and to add insult to injury, it was Maguire who headed home the clinching goal for United.
Chelsea's victory over Spurs at Stamford Bridge on Saturday proved the first time the officials had admitted a mistake in a VAR ruling.
And that left Tottenham manager Jose Mourinho frustrated with what he considered an unwanted precedent, calling on officials to be consistent with handling mistakes.
Mourinho continued to lament Liverpool's Andy Robertson escaping punishment for fouling Japhet Tanganga in the Reds' 1-0 win at Spurs on January 11, and Etienne Capoue avoiding any sanction in Tottenham's goalless draw at Watford a week later.
"I hope the noise is the same noise as when the VAR kills us," said Mourinho.
"I hope the noise is exactly the same; against Liverpool when Robertson should get a red card and didn't. Against Watford when Capoue should get a red card, with the same gentleman here, Michael Oliver.
"Why didn't they say that when they made a mistake against Liverpool and Watford? That's what I mean by the same noise."
Spurs boss Mourinho claimed an inside source had leaked him Chelsea's plans to switch to three centre-backs and wing-backs ahead of Saturday's derby clash.
Lampard admitted he fully believed a source had passed on the information, but he was phlegmatic about the realities of modern football.
"That's the world we live in; I'm not being sinister about it, but that's the world we live in," said Lampard.
"I'm not that trusting in all ways in everybody. That's life."
Mourinho meanwhile insisted after his side's loss that he had only guessed at Chelsea's tactical switch, and had not in fact been leaked any inside information.
Asked about his predictions, Mourinho said: "I guessed; because there is a logic, there is a logic on it.
"When they have a run of bad results they go to five (in defence). That's something obvious.
"So nobody leaked me anything. They are loyal to him (Lampard)."