Mark Hughes felt the first half sending off of Gelson Fernandes changed the course of Sunday's clash at Eastlands.
The midfielder was sent off for two bookable offences - fouls on Luka Modric and David Bentley - with only 26 minutes on the clock.
Darren Bent levelled Robinho's opener soon after and the same player struck for victory after 64 minutes.
City boss Hughes said: "Up to the sending-off, we were very much in the ascendency and comfortable with the way the game was panning out.
"Obviously we were disappointed with the sending-off, the first booking was valid as Gelson had slid into the back of their player so by the letter of the law, that's a booking.
"With the second one I felt that the referee had not any stage of the game given any players the benefit of the doubt with consideration of the conditions out there. I think Bentley nipped the ball past Gelson, there was the barest of contact and I don't think Bentley was actually in control of the ball when the slight contact was made.
"These things change games and then we conceded the first goal, it was a straight ball down the middle that squirmed under Richard's (Dunne) foot, so they took advantage and equalised.
"We had to remain positive even when we went down to 1-2, and we worked hard all over the pitch when down to nine men. The qualities we showed augurs well for the future, the lads showed some real character."