Kevin de Bruyne threw a strop during Manchester City's Carabao Cup win over Rotherham.
City's players are required to watch matches from the bench when they are taken off unless they are injured, which De Bruyne was not when he was replaced by Phil Foden after 58 minutes of Wednesday's win.
The Times says De Bruyne headed straight to the dressing room because he was upset at not getting more minutes under his belt having endured an injury-stricken season so far.
"I didn't know he was in the locker room," Guardiola said after the match.
The game against Burton was only De Bruyne's fourth start of the campaign after sustaining ligament damage in both knees during 2018-19.