MANCHESTER, England -- Pep Guardiola has confirmed his altercation with John Stones on the Manchester City bench was because the defender was not ready to come on.

Stones was called on to replace the injured Rodri in the 41st minute of the 5-1 win over Atalanta in the Champions League on Tuesday.

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Guardiola appeared frustrated with Stones before he took to the field and the City boss has reminded his substitutes they are expected to be prepared whenever they're needed.

"The guys on the bench know you have to be ready to go out from the first minute," Guardiola told a news conference on Friday.

"Sometimes it's the t-shirt or other issues. Did you see the image? So it speaks for itself."

Stones hasn't started a game since the defeat to Norwich in September because of a thigh injury.

The 25-year-old is in line to start against Aston Villa at the Etihad Stadium after Rodri, who has started the past two games at centre-back, was ruled out until the next international break with a hamstring problem.

Oleksandr Zinchenko is also set to miss six weeks with a knee problem as Guardiola continues to deal with mounting injury list.

"We are in a bad period in that sense," said Guardiola.

"I'm a little bit concerned because in a few positions we don't have enough players.

"I would prefer all the players had no injuries but I have to do it, yes. We can't start complaining, 'oh what bad luck.'

"We have good players in the squad. In a few positions there are not many but we have incredibly good players. We're going with them."