Stones has emerged as an important option for City in central defense after struggling last term.
He was excellent against Olympiacos in the Champions League in midweek, and continued that form in the team's comfortable Premier League win over Burnley on Saturday.
"He played two already," Guardiola told reporters.
"The previous season it was difficult to feel it - sometimes for my decisions but most of the time because of injuries.
"It's good again because he was so concentrated and didn't make any mistakes. What we are looking for from the back four is to not make mistakes."
Asked how Stones was reacting to more competition for places, Guardiola added: "It's not too complicated to understand. I see the performance and the levels, and the guys that help the team and help me have more chance to play.
"The other ones have to wait for the opportunity and they have to fight and wait for their moment and, when they have the moment, do it as well as possible.
"This business it works in this way, not any other way. What happened one week ago or today for example is past. Now you have to do it in the game in Portugal."