The French striker was again barracked by Real supporters during victory over Las Palmas on Sunday.
Asked about Benzema, Marcelo stated afterwards: "I'm not going to talk about injustice because here, all the players have already been whistled.
"If the fans whistle you, you have to improve yourself. It's difficult to play in these conditions but you have to train as Benzema does, fight ...
"Whistles are part of football."