"He was a real handful playing up front with Forlan, he looked very sharp. Even then he looked like he would go for big money, and that's what happened," Smalling told The Sun ahead of Sunday's clash with Aguero's Manchester City.
"Since he arrived, he's really produced for City, scored some great goals and some very important ones.
"We'll have to keep an eye on him but hopefully it will be me celebrating this time.
"Getting revenge against him would be nice. I'm looking forward to trying.
"But it's not just him, we can't think too much about him when there's players like Silva and Nasri around, too."