Villa went behind when Ashley Young theatrically fell over Ciaran Clark's challenge and after Wayne Rooney converted from the spot, there was no way back.
"I didn't think it was a penalty," he said.
"Ashley threw his leg into Ciaran Clark's leg and Ciaran was static. He planted his feet firmly on the ground. He then tries to pull his leg away.
"Some other people may disagree but I felt it was a very soft decision."
Sir Alex Ferguson has admitted Young might have emerged from the past seven days with a reputation as someone who goes down too easily - however, the United boss claims never to have seen any evidence he is a diver.
Seven days after Young was embroiled in a spot-kick row against QPR, he was at the centre of more controversy against former club Aston Villa.
Referee Mark Halsey decided the contact as Young attempted to step inside Clark just six minutes into the Old Trafford encounter was worthy of a penalty.
Ferguson said it was a penalty, but conceded Young had gone looking for it and, when asked directly whether the winger might be looked at as someone who goes down too easily, Ferguson responded on MUTV: "I think in that situation, in the last week or two yeah. I have never seen that in him. It is not an habitual thing with him. He was brought down. He just made the most of it.
"But he definitely brought him down. There was contact. He has bought the fact that Ashley jinked inside him.
"He has gone down quite easily but it was a penalty. He was definitely brought down."