Mascherano was shown a second yellow card and dismissed just before half-time after abusing ref Steve Bennett and his assistants no less than SIX times in a non-stop tirade at Old Trafford.
He then refused to leave the pitch and had to be restrained as he tried to confront Bennett.
He explained: "I don't know why he got involved. He came across from central midfield to argue with the referee.
"With what happened during the week with the Ashley Cole thing, there's been a focus on dissent. He'd already been booked and he kept badgering the referee. Dissent is dissent - and I think that was dissent."
Liverpool boss Rafa Benitez amazingly tried to defend his player. The Spaniard said: "He knows he made a mistake but in this kind of game it is wrong to leave a team with just 10 players because one of them asked why.
"I talked to him in Spanish from the touchline and I kept telling him to stop, saying 'It's finished'. He doesn't know the referees so well and I wanted him to stop.
"Mascherano is a fantastic professional and has been playing at the maximum level for years. When you play a big game like this you are surprised by something like this. And you want to know why you are sent off.
"We are talking about players who are passionate and who want to win. Mascherano was trying to work really hard, like everyone else, and we are disappointed with what happened.