The Argentina striker has cemented himself as a City hero thanks to his exploits in nearly five years at the Etihad Stadium, most notably scoring the last-minute winner against QPR in 2012 which delivered the club's first Premier League title.
Despite struggling with injuries this season, he has scored 11 times in 20 appearances in all competitions and Sagna believes the Argentine has a unique blend of talent.
"He has that difference to make him a top striker," he replied when asked if Aguero is similar to his former Arsenal team-mate Henry. "He's unbelievable. To me he is a mix of Henry and Messi.
"In smaller spaces he manages to keep the ball well. He showed against Norwich that he can be quite tricky and selfish to score that goal.
"He is a key player. He's done it a lot in the past and he can do it [now]."
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He had been back in action for just a week when he limped out of City's game against Southampton at the end of November, which condemned him to another month on the sidelines.
But his fine individual goal against Norwich City in the FA Cup last weekend was a sign that he is returning to full fitness and Sagna believes City's other attacking players need to do more to share the goalscoring burden.
"Some players will say he doesn't score enough this time but other teams aren't stupid - they give him more attention," he added. "They bring more players around him.
"We have to find another solution to help him. Maybe we have to step up and players have to show more character to try to score themselves."