The England defender appeared to kick out at Michy Batshuayi in the first half – but escaped punishment after a VAR review.
Lampard said it was a decisive moment of the 2-0 loss – and claims it was just wrong.
The Chelsea boss was also frustrated after Kurt Zouma had a goal ruled out by VAR after Cesar Azpilicueta was adjudged to have fouled Brandon Williams in the build up.
He said: “Harry Maguire should’ve got a red – then he scores the second goal and the game changes off that.
“That’s a major part of what VAR was brought in for. That one was wrong.
“I didn’t have a dialogue (with the fourth official) – I hadn’t seen it, so there was nothing to say.
“It was just the wrong decision. It’s harder to take with the presence of VAR. That’s the reason it was brought in for, love it or hate it.
“It was universally the wrong decision. It’s not my opinion. Everyone I’ve spoken to says the same thing.”
But Ole Gunnar Solskjaer disagreed with his Chelsea counterpart: “For me, I didn’t think two seconds about it at all,” he said.
“I think he was fouled, rolled over, seems like Batshuayi was gonna land on top of him and he puts his leg out to protect himself. It hurts when you get hit there! But I think he’s all right.
Goals from Anthony Martial and Maguire secured victory for United and moved them to within three points of fourth-placed Chelsea.
But it could have been a different story if Zouma’s strike hadn’t have been ruled out.
“I think Zouma’s goal should have stood,” added Lampard. “At first I didn’t. It’s certainly not clear and obvious.”