Mancini was furious at the limp manner of City's 3-1 defeat at Southampton at the weekend which now leaves the reigning champions 12 points behind leaders Manchester United in the Barclays Premier League with 12 games left.
Mancini said he will make significant changes for the FA Cup fifth-round visit of Leeds on Sunday, telling the Manchester Evening News: "I am sure we will change that, because I will change players next week. I only want players who are ready for the fight in the last 12 games."
He continued: "I am very angry with a lot of my players, and very disappointed at the performance, because it is impossible to play the way we did.
"We didn't fight for every ball. We can't always win by playing well - that is normal. Sometimes we play well and win, but you also need to be able to win because you fight for every ball, and fight against an opponent who also wants to win the game.
"Every team that plays against us treats it like a Champions League final, but we should know this, it is normal.
"We now have 12 games left, (we) need to do well in the Premier League, and we have the FA Cup - there are games left and we can do better. We need to finish the season the way we did last year."
Carlos Tevez is expected to return to City this week after a family problem, and Vincent Kompany should also be available again, as will Matija Nastasic and Kolo Toure who were not risked after late returns from international duty.
Mancini branded the fixture demands caused by the international fixtures "ridiculous" but did not hold his defenders - including midfielder Javi Garcia in a makeshift centre-back role - responsible for the Southampton defeat.
He added: "The fixtures are ridiculous sometimes, as we played on Saturday and then had six players who came back on Friday, and five who came back Thursday night. Sometimes it is impossible."
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