The Argentina international opened the scoring for the Citizens in South Wales with a smart finish past Neil Etheridge from his side's first shot on target.
That strike took Aguero to 205 goals for City in all competitions since arriving from Atletico Madrid in 2011, and Guardiola insists that there can be no doubting the 30-year-old's iconic status at the Etihad Stadium.
"He is an authentic legend," he told Sky Sports News. "It's important what he has done at this club and hopefully we will have him for longer and he stays in a good position.
"The level today was the level of last season. We cannot forget the Champions League, the opponents are good too, we knew that. It's part of the process.
"The season is long, there are a lot of games to play. After one not good result, the team came back where it normally is. Of course we are more confident at 0-3 than 0-0, to score goals you have to be patient and let the game speak for itself."
Aguero signed a contract extension earlier this week that will run through until 2021.