Aguero who is among the biggest talents in the world, could have joined Liverpool when he was 17-years-old in 2006.
The Argentine joined Atletico Madrid for £14million instead.
“Two years ago the agents were talking with us about players and he was one of those but he was too young,” Benitez told The Times.
“Atletico paid big money for him, around 18million euros [about £14million] for a player who was a risk.
“In the Premier League it’s different and sometimes players will need more time here to improve.”