The Argentina forward threw himself into a tackle on the Brazilian with both feet off the floor in an incident that went unseen by referee Chris Foy.
City eventually won the game 2-1 to set up a final against Wigan Athletic next month, and Platt has said that no action should be taken against the forward.
"I don't think we should be analysing or re-refereeing games on a Monday morning. If you want to do that, you would have to analyse the whole 90 minutes of every single game," Platt is quoted as saying in The Express.
"The rules are the rules, we haven't made them, we just adhere to them. In my opinion there are a lot of bullets flying at Sergio and, as an employee of Manchester City, he needs someone to stand with him in the trenches and face those bullets. That is where I am."
Aguero has since apologised for the incident.