England team-mate John Stones has been favoured in recent weeks in an inverted full-back role where he pushes into central midfield alongside Rodri.

"He (Walker) cannot do it. To play inside you have to make some educated movements," said Guardiola.

"He has other characteristics. He will always have pace. He will be the fastest in this room at 60 years old."

Defending Premier League champions City, who are second in the table, face Southampton at St Mary's at 17:30 BST on Saturday.

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Joao Cancelo and Oleksandr Zinchenko played the inverted role, which allowed Walker, who has 75 England caps, to slot in on the right of a back three when they pushed into midfield.

But Zinchenko left for Arsenal last summer and Cancelo joined Bayern Munich on loan in January.

Nathan Ake is now playing regularly at left-back and drops into the back three, meaning the right-back is required to push forward, something Stones did to great effect in last weekend's 4-1 Premier League win over Liverpool.

"The reason why is tactical. It's not because we lost faith in Kyle," said Guardiola.

"He can play coming inside and he has done it many times, but like in the position [against] Liverpool with Rodri and John, he will maybe need time to do it and we don't have time."

Guardiola offered Walker hope of a return by referring to Ake's progress, and urged every player in the squad to be ready to play.

"What they have to do is two things," Guardiola said. "Don't complain or there will be a problem. And try to do it better and better so when I need them, they will be ready.

"Look at Nathan this season. Last season he didn't play one minute and now he is undroppable. It happens.

"Nobody has a guarantee with me. They have to earn it and sometimes we need a different shape for the way we build up or defend. Some players adapt better than the other ones.

"They take it personally but we are not against them. We just need something and try to use it."