Aguero isn't the type of player who can come into a team and transform it, neither was Diego but they are both really good players and by having a good combination of players lets say Diego, Krasic, Aguero, together they can change the team a lot!
We don't need to sign game changers like Messi Ronaldo etc just sign players capable of doing their job properly. We need to get rid of useless shits like Martinez and Motta and sign A good Centre Mid, LB, Left Wing and CB instead of spending it all on Aguero