The Argentine midfielder was dismissed just before half-time in Sunday's 3-0 defeat at Old Trafford for a second yellow card.
Mascherano, who had earlier been booked for a foul on Paul Scholes, was cautioned a second time for showing dissent towards referee Steve Bennett.
Mascherano has now expressed his disbelief at Bennett's decision and insists he did not swear at the official in the incident which saw him dismissed.
"I do not know why I was sent off," Mascherano told the Liverpool Echo.
"I asked the referee what was happening. I did not swear, I was not aggressive and I did not confront him.
"All I did was ask him what was happening, nothing else.
"So when he showed me the second yellow card and sent me off I could not believe it.
"I am sorry to my team-mates because that meant we went down to 10 men and that made things even more difficult for us."