Sergio Aguero was forced to withdraw from Manchester City's squad that was to face Birmingham inorder to self-isolate.
The striker was expected to feature as City hosted the Championship side, but missed after someone close to the striker tested positive for coronavirus.
The striker's test however came back negative but was nevertheless left out as per the guidelines.
"Unfortunately he was in contact more than usual with one person who is positive," Guardiola confirmed after the game.
"He must isolate for a few days. I don't know exactly because he has been tested like all of us six times in the last 10 to15 days so it is negative.
"Today, I think he was negative too but the protocols say in some cases isolate, and in other cases, we still play football. He was not able to play today."
The striker's season has been heavily affected by a niggling injury that saw him get an operation last year, limiting him to just three starts this season.
Guardiola has been careful with his recovery and has time and again insisted they will not rush him back.
The 32-year-old might miss City's next two matches; against Brighton on Wednesday night and Crystal Palace on Sunday.