Wolves boss Gary O'Neil says elite referees manager Jon Moss told him he "could not believe" his side were not given a penalty at Manchester United.
The home side won 1-0 at Old Trafford thanks to Raphael Varane's header.
O'Neil was furious his team did not get a late penalty when United keeper Andre Onana crashed into Sasa Kalajdzic and failed to make contact with the ball.
"Jon Moss said it was a blatant penalty and should have been given - fair play to him, he apologised," said O'Neil.
"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.