Forest lost 3-2, with Swans scoring twice in injury-time to deliver a second defeat in Postecoglou's first two games in charge.
Afterwards, BBC Sport is reporting Postecoglou could be heard blasting his players before speaking to the media. While details weren't offered, the Australian was heard shouting at Forest's players in the away dressing room immediately after the final whistle.
Later, when speaking with the attending media, Postecoglou said: "It's cup football. We should have been a lot more prepared for what was going to come and handled it a lot better than we did. It's not about me.
"It's about the club. It's about us progressing and creating an environment where we can have real success."
