Of the last seven matches Manchester City have played, Walker has only started twice, with central defender John Stones preferred at right back.
Guardiola has deployed Stones in an inverted full back position which requires the former Everton defender to drift inside to join Rodri in midfield.
Speaking ahead of City's Premier League trip to Southampton, Guardiola said: "[Walker] cannot do it. To play inside, you have to make some educated movements."
"He has other characteristics. He will always have pace. He will be the fastest in this room at 60 years old."
The City manager added: "The reason why is tactical. It's not because we lost faith in Kyle.
"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."
"What they have to do is two things," he 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 [Ake] 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."
Walker has made 23 appearances in all competitions this season.
