The versatile defender has warned his side they must make his compatriot "uncomfortable" at Camp Nou and insists they can still win the treble
The prolific forward missed City's 2-0 defeat in the first leg due to a hamstring injury but has since shaken off his physical problems and played in the weekend defeat to Wigan.
Mascherano has acknowledged his side must make the Argentina striker "uncomfortable" at Camp Nou if they are to secure passage to the quarter-finals.
"I think tomorrow’s match will have Man City on the attack. They will try to use lots of attacking players," Mascherano said at a press conference.
"I know Sergio Aguero and he's one of the best five forwards in the world.
"He is capable of ruining your day so we will try to not give him much space, make him uncomfortable."
Mascherano then went on to discuss Barcelona's ambitions for the remainder of the season and stressed that the Catalans want to win it all.
"Obviously the motivation is to win. At the beginning of the season, you imagine how things will develop during the season and how it will unfold.
"Be in the Champions League final, fight for the league title, and so on. At the moment we have the chance of doing something great, it's time to push the accelerator and give our all."