The German international's joy at scoring a spectacular free-kick on his second start for 13 months evaporated as City mounted their unlikely second half revival from 3-0 down.
"I just didn't think we would lose the game - never, never, never in my mind," said Christian. "It has been hard to come to terms with, but things do happen in football. They had ten men and there are no excuses for it, losing from 3-0 up.
"It simply wasn't good enough for us, but the key is now to leave it behind and get on with the game against Portsmouth - especially for the players.
"I have had setbacks in my career and you must look forward. Tomorrow is a good possibility for us, we play Portsmouth, it is a big game for us and the chance to get three points at home."
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Christian Ziege says he struggled to come to terms with the FA Cup defeat to Manchester City on Wednesday as preparations step up for the return to Premiership combat against Portsmouth tomorrow.