Despite a positive start the Reds went a goal down on 33 minutes, then a rash challenge by the Argentinian midfielder saw him seeing red and the team down to 10 men.
A late goal by Pompey ensured victory for the home side and left Liverpool five points behind fourth-placed Aston Villa, having played a game extra.
"If you analyse the first half we did well," said Benitez after the match at Fratton Park. "We conceded a goal and the sending off of Mascherano changed everything.
When asked about Mascherano's injury which he picked up during the sending off incident, the Liverpool boss added: "We will have to wait, maybe it was his ligament that he damaged so we will have to wait until tomorrow.
"Hopefully we can win the next game. I think we had some chances that maybe we could have scored. We were controlling the game but everything changed with the sending off of Mascherano."