Former City striker Leroy Lita scored twice, the second a penalty after David James was dismissed, while Kalifa Cisse scored an own-goal and Barry Robson was also on target.
Millen emerged to say: "We thought we had had a wake-up call against Sheffield Wednesday in the FA Cup last week, but this was another one.
"When we don't play at a high tempo we are not a good side and our passing in the first half and overall play was far too slow.
"It was a low point in the season for us and we need to pick ourselves up quickly because the last two performances have not been anywhere good enough.
"We shall be working hard on the training ground before our next game and need to look a lot sharper."
Source: PA
Source: PA