Prolific striker Aguero is City's chief attacking threat and has scored 28 goals in 41 appearances in all competitions this season.
City will rely heavily on the former Atletico Madrid star if they are to beat Casemiro's side having drawn last week's first leg 0-0, but the Madrid man says the visitors are by no means a one-man team.
"They've not only got Aguero, they've got big players with a lot of quality," Casemiro told Bwin's website.
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"We have to be switched on to all of them and focused on the game. Play the same as we did there, with attitude and character.
"We have to play with a lot of attention and strength."
Madrid defender Pepe is set to be tasked with shackling Aguero, and Casemiro added: "We know that Pepe is a great player.
"He's been here a long time, he's very experienced and one of the best centre-backs in the world.
"We count on him because he is a player with a lot of character and a lot of energy out on the field."