Hosting Derby County at St. Andrew’s Trillion Trophy Stadium, Blues started competitively but found themselves 3-0 down at the break before the visitors added a fourth in the second-half.
And the Head Coach had no time for hard-luck stories after the loss, sharing the disappointment of fans watching from home.
He said: “I agree with them in their critique and everything they say about me and us, they are completely right.
“For the supporters: I apologise. The second thing is to keep believing in this team.”
The setback comes three days after Blues ended a losing run following a resilient performance away to Nottingham Forest, with Karanka evidently agonising over the undulating fortunes of his side.
Speaking to the media he said: “It is difficult to understand and difficult to accept. It is something that is not every game.
“It is more difficult when you do this one game and then one game you are perfect. The frustration is that I don’t understand this situation."
The Boss also told the press that he is not looking for excuses following a testing December.
He explained: “Maybe it is a mental thing to play at home without a crowd, but I don’t want to make excuses because we played really well in the first home game and against Huddersfield.
“It is about correcting the mistakes as they are individual mistakes.”
Blues return to action on Saturday when they welcome Blackburn Rovers to the West Midlands for a 3pm kick-off.