Benitez said: "Javier is a fantastic professional and he knows he made a big mistake, he apologised to his team-mates and the supporters and he has now apologised to the referee.
"He accepts he lost control after the sending-off and has said sorry to the referee. I have also talked with him now and told him he would be fined by the club, and he has accepted that and wants the money donated to charity.
"We would like a personal hearing now. This was the first red card of our season and we do not have problems of this kind. We will now await the outcome of the hearing and the lawyers are now working on it."
Mascherano will serve an initial one-match ban in the Merseyside derby on Sunday.
If the charge of improper conduct is upheld, Mascherano looks set to be in line for an additional suspension, probably for two matches, which would see him also miss Premier League games against Arsenal on April 5 and Blackburn eight days later.
"The charge relates to his reaction towards referee Steve Bennett after being sent off against Manchester United on Sunday, March 23.
"The case will be heard by a regulatory commission on Thursday, April 3.
"Mascherano is currently serving a one-match suspension for the sending-off."