"We've left everyone flat," he told whufc.com.
"The fans deserve better than that and we certainly can't carry on like this. It's a massive disappointment to say the least and now we've got to go and play a lot better at Birmingham.
"Unfortunately, we didn't show any of the things that we've been working upon in pre-season and we looked laboured and tentative and didn't put Manchester City under any pressure.
"It's certainly given us something to think about. When I looked at the statistics after our pre-season win at Norwich City 11 days ago, they were the second best since I've been at the Club but we started off poorly and just got worse as the game went on.
"Robert Green was the only one who came out with any real credit and the rest of the players have got to look at themselves because we just couldn't string two or three passes together."
He added: "The crowd were at full throttle but it turned out to be a massively disappointing game for the players, the fans and everybody connected with the club.
"It's only come after our first game, though, and now we've just got to get on with it and look forward to putting things right next week."