Mascherano appeared to question referee Steve Bennett's decision to caution Fernando Torres at Old Trafford and was then shown a second yellow card himself for dissent.
The Argentine, who admitted the charge, initially refused to leave the pitch.
Mascherano had to be calmed down by Reds manager Rafa Benitez before he relented and left the field.
The midfielder, 23, was initially fined two weeks' wages by his club and banned for one game by the FA.
Mascherano has since apologised, telling the Daily Mail earlier this month: "I made a mistake and I regret this mistake. It was my fault. I apologise to everyone.
"I reacted badly to the red card because I could not believe what was happening to me."