Jose Mourinho's Chelsea side have suffered only two top-flight defeats and will take some stopping as they remain in the hunt for three trophies.
Second-placed City suffered a setback when they lost at home to Arsenal last month and last week's 4-1 hammering of Stoke City was their first victory in five Premier League games.
Aguero, who struck twice in that win, is still confident City can retain the title and does not believe the champions are in a two-horse race with Chelsea.
When asked if City can win the title, the Argentina international told the club's official website: "Why not? We were worse off in past seasons we still won and I was always convinced we'd be champions.
"There is a long way to go and a lot of games to play so anything is possible. Right now, it's anyone's title and it's not just two sides competing - there are many teams that are still in the hunt.
"Arsenal and United are back on their feet, Southampton are not far behind and Liverpool and the Spurs have all shown good form, too.
"I believe this will be one of the most highly contested Premier Leagues of recent years and we will be giving everything to retain our title."
Manuel Pellegrini's side host Newcastle United on Saturday, with Chelsea taking on struggling Burnley at Stamford Bridge.
