The Argentinian midfielder saw red shortly before the interval after recklessly crashing into Tal Ben Haim, although his feet did not leave the ground.
Benitez appeared to be upset with the referee's decision to give Mascherano his marching orders and when asked his opinion on the referee's performance, he sarcastically replied "perfect".
"I have seen the replay and for me it is not a sending off but anyway, the referee was perfect, he didn't make any mistake," Benitez said on Sky Sports.
"Now Javier is injured and I will say again the referee didn't make any mistakes. He was perfect. I have seen the replay, for me it is not a sending off, but anyway, perfect." If you analyse the first half we were doing well, even with their goal but then came the Mascherano incident."
"We were a good team in the first half and the sending off changed everything. We had some chances, we should have scored, we were controlling the game and everything changed with Mascherano. Now we have to hope we can win our next game.''
Pompey's victory saw Avram Grant maintain his unbeaten record against Benitez after posting two wins and two draws during his time at Chelsea in the 2008 season, and the result reignites the club's hopes of survival.
Grant was thrilled to claim the scalp of Liverpool having narrowly missed out to leaders Chelsea in midweek.
"Today we played like a team,'' he said. "From game to game we are playing better and today we deserved the win.
"We played on Wednesday against a team in first place at home, and we showed ourselves if we can do things well at the bottom of the table we can still surprise the other team, and today it went well.''
Pompey midfielder Michael Brown added: "We've been playing well in general. It was a fantastic result for us. It's a matter of getting some points and three points is fantastic against a strong Liverpool team with a lot of talent. It's an enjoyable day for us.''