Aguero has struck eight times in his last seven outings for the Blues, and Robson fears that failure to shackle the Argentine could prove fatal at Eastlands.
City go into the game without a win in three games, and the Red Devils legend is well aware that his former club have more than enough firepower themselves to pile more misery on their stuttering neighbours.
"I think the defenders are going to have to do really well against Aguero, if he is fit – that would be a plus if he is out," Robson told MUTV.
"Dzeko is a real handful and Silva, I have been told he is probably not going to play – that is also a plus. So if we can defend really well, it has been shown that City concede goals.
"From being a really top team and being champions, they can concede goals and we have got the strikers who can do that."
Kickoff at the Etihad Stadium is scheduled for 1.30pm, with United currently four points behind City in the table.