The boss said he had no plans to make the change, particularly as Joao Cancelo had looked dangerous at left-back in the first half, but an ongoing problem meant Walker could not continue.
“I made a change because Kyle Walker could not continue,” Guardiola explained.
“In the first half all the dangerous situations we created were from Joao [Cancelo].
“Kyle had an infection in his stomach in the last weeks. He didn’t train and lost weight. He made an incredible effort today because we needed fast players.
“At half-time he said he didn’t feel good, that’s why we made a substitution.”
Watch what the boss’ verdict on the defeat in the video above.