Alex McLeish lamented Blues 'flat' performance that brought to an end a healthy run of form in the Premier League.
And the Blues boss now has to contend with worrying injury problems ahead of the Carling Cup final a week on Sunday.
Newcastle United ended a five-game winless run, having got off to a flying start at St Andrew's that angered Big Eck.
"We have had a fantastic month, in terms of results, after the Man United game," said McLeish, in reference to Blues return of seven points from three games.
"There has been some excellent performances, inspirational, heroic. But we just looked a bit flat.
"The first goal, to lose it the way we did, and so early, goes against exactly what I thought about the team at the weekend, when the concentration level was phenomenal from start to finish.
"I don't think you can underestimate the fatigue. My experience in the game tells me that looking at them, there were one or two who looked a wee bit tired, off the pace.
"But we could still have avoided the goals, certainly the first one was very avoidable.
"My parting shot to the players was 'start fast, get Newcastle on the back foot'.
