Benitez was involved with a heated touchline exchange with Ferguson after Wayne Rooney's converted spotkick.
However, the Spaniard did not believe Webb had been influenced by Ferguson's comments prior to the game.
"Some decisions in the past have not been the best for us but I consider Howard Webb a good referee," he said. "The referees are professional. We know the influence of Sir Alex in everything but he is a good referee.
"Jamie Carragher was coming inside so he was in the middle of the action (and therefore Mascherano should not have been red-carded)."
Of their touchline exchange Benitez said: "When you have different opinions you have to express them. He has his own opinion about everything."