Mascherano has admitted that Aguero could make his life a nightmare as Manchester City chase down a 2-0 Champions League deficit in the Nou Camp.
Mascherano said: "I know Sergio well and in my opinion he is one of the best five strikers in the world.
"He is a great player and if he is given space he can be a nightmare to play against.
"We will try and give him as little space as possible as the last thing we want is for him to be comfortable. We need to make things uncomfortable for him.
"I think we will see a very attacking Manchester City team. They always attack but the result from the first leg means that they will throw even more players forward.
"We will try to dominate the possession. That's the only way to play against really good teams."