"Jon Moss said it was a blatant penalty and should have been given - fair play to him, he apologised," said O'Neil after the match, which United won with Raphael Varane's 76th-minute header.
"I have spent a lot of time with him today to understand the new guidelines, trying not to get myself booked in the first game, which I have failed in.
"But fair play to Jon for coming out and saying it was a clear and obvious error - he couldn't believe the on-field referee didn't give it and can't believe VAR [video assistant referee] didn't intervene.
"It probably made me feel worse, actually, because you know you are right. I feel worse about leaving with nothing. Live, I was told they didn't think it was a clear and obvious error."