The reigning Premier League champions suffered a demoralising loss at the hands of their local rivals at the Etihad Stadium, as a Sergio Aguero double and further goals from Yaya Toure and Samir Nasri did the damage.
And, despite Wayne Rooney netting a late free-kick to ensure Moyes' first Manchester derby did not end in complete humiliation, the Scot was at a loss to explain his side's lacklustre showing.
"I'm disappointed we didn't perform because there was no reason," he said.
"We'd played very well in midweek (against Bayer Leverkusen), there was no reason for us not to perform well today. But it's one game, there are plenty more to come and there is plenty of time for us to fix it.
"I thought we were second to things, in the first half especially. We seemed to be a yard behind it and never really got to the pitch of the game which was disappointing because we had set out to do that.
"We had a good result in midweek to build on and come into this game but we never really got to grips with it."
The former Everton boss was insistent, however, that his squad will bounce back ahead of Wednesday's League Cup clash with Liverpool at Old Trafford.
"I think if there was ever a group of players who I would expect (a reaction) from, it would be Manchester United players," he added.
"The way they've been brought up with the manager previous, they react. And that's what we'll do here, we'll react. Every manager has bad days and bad results, I'm no different."
