The Spanish manager should have been celebrating what was a crushing psychological blow in Munich’s favour in the context of the title race.
Instead, he was left to snarl like a mafia don taxed by a rat in the family.
Apparently, there is a mole with Bayern’s ranks, who has been leaking team information to the media.
The suspicion is that the mole is a player, and Guardiola is furious.
He says that when he finds out who the culprit is, that player may have kicked his last as a Bayern player under him.
"It does not matter who it is, heads will roll," Bild claims Guardiola told the Bayern squad ahead of their win over Dortmund, before which his tactics had been accurately reported on.
"I will throw him out. He will not play under me again."
Bayern’s CEO Karl-Heinz Rummenigge has confirmed the severity of the situation, but has attempted to take a slightly more diplomatic line than Guardiola.
"We will obviously not bring in the NSA (National Security Agency) to find out who it is," Rummenigge told Sky television.
"But I advise him to stop doing it or he will have a serious problem not only with Pep Guardiola but also the entire club."
Things have gone smoothly for Guardiola since he took over the Bayern job from Jupp Heynckes this season.
The Bavarian juggernaut rolls on, and only a penalty kick mutiny by Arjen Robben has caused the Spaniard a brief moment’s discomfort.
With Bayern flying high in the Bundesliga and Champions League, looking every bit the treble winners they are, the last thing they need is a ruction in their camp to destabilise them.
Who do you think is the Bayern rat? Tell us in the comments section below.