Aguero, who arrived at the Etihad Stadium from Atletico Madrid in the summer, has scored over 20 league goals in his first season at the club despite the difficulties in adapting from one league to another.
"I've known Sergio for a long time," he said.
"I knew before he came here that he was fantastic so I'm not surprised how well he has done. He has adapted into the team, he's a great person to have here.
"La Liga is completely different to the Premier League. But Sergio has been one of our key players."
The Argentinian has bagged 21 goals in 27 league starts for the Citizens.