Nasri, 29, was an unused substitute for Thursday's International Champions Cup win over Borussia Dortmund in China.
Defender Gael Clichy revealed on Wednesday that Guardiola had exiled some players from first-team training until they meet certain weight targets.
And the new City boss said of Nasri: "He arrived a little bit overweight."
The France international missed six months of last season with a thigh injury he suffered in training.
Guardiola, who arrived at the club at the start of July, said the player's fitness is "much better now" and added: "Last season, he was injured in the season. We want to avoid that. We want the players on the pitch to be fit.
"We want their weight to be in the right place where they can run 90 minutes like Fernandinho and [Aleksandar] Kolarov and the others. Hopefully in the next two weeks, he will be OK."
When asked if he was pleased with Nasri's progress, Guardiola said: "I cannot have any complaints about the team. It's the opposite.
"The fans can be so proud of these players. In every training session, they've tried and tried with the rhythm we play."
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Manchester City midfielder Samir Nasri has not yet played for the club during pre-season because of his weight, says manager Pep Guardiola.