Kevin De Bruyne chose to sit in the dressing room rather than watch the remainder of Manchester City's 9-0 victory over Burton following his second half substitution.
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.
According to The Times, 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.
He opened the scoring after only five minutes of the Carabao Cup semi-final at the Etihad Stadium, his third goal of the season.
Should the Belgium playmaker start in Monday's match against Wolves at the Etihad, it will only be his second Premier League start of the season.
City are looking to dislodge Premier League leaders Liverpool, who they currently trail by four points in the standings.
The return leg against Burton at the Pirelli stadium is on January 23.