City have been hampered in their trophy quest this season by five red cards, three of which have come in games they failed to win.
The latest of them has been the most damaging with captain Vincent Kompany now facing a four-match suspension for his dismissal in City's FA Cup loss to Manchester United on Sunday.
There have also been three other sendings off in the last 11 games - for Mario Balotelli, Gael Clichy and Gareth Barry - and Mancini did not agree with any of them.
The Italian said: "This is strange. In England now there are a lot of red cards for nothing.
"Vinny did nothing. Usually they give a red card for nothing - for Clichy against Chelsea, for Gareth Barry. I do not understand this."
City have lodged an appeal against Kompany's straight red card, shown for a two-footed challenge on United's Nani, and are hoping to have the ban overturned at a hearing on Tuesday.
Should they fail, the influential 24-year-old's suspension will begin with Wednesday's Carling Cup semi-final first leg against Liverpool at the Etihad Stadium.
He would also miss the return leg at Anfield as well as Premier League games against Wigan and Tottenham.
Mancini at least felt he learnt a lot about his side from how they performed with 10 men against United.
The Belgian was sent off in just the 12th minute, when City were already 1-0 down, and they were trailing 3-0 by half-time.
But they produced a rousing reply after the break and had United clinging on before their rivals eventually edged them out 3-2.
Mancini said: "We think if we had 11 players at 1-0, we wouldn't have any problems.
"I am proud of the guys, with their performance. We made one or two steps forward.
"We lost the game but we improved a lot.
"I think we did very well. If we have this attitude for all the season we can win cups."
City still have a three-point lead at the top of the Premier League but the loss of Kompany would be a major blow in the coming weeks.
Kolo and Yaya Toure are already unavailable due to the African Nations Cup while strikers Edin Dzeko and Balotelli missed the United clash through injury.
Dzeko could return to face Liverpool after a knee problem but Balotelli's ankle injury is likely to keep him out.
Midfielder Barry is available again after a one-match ban.
