The City boss was furious with his team after their 3-1 defeat at Southampton as they fell 12 points behind Manchester United in the Premier League title race.
And Mancini insist he will only select players he believes are ready to roll their sleeves up for the cause.
"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 told the Manchester Evening News.
"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 should be available against Leeds after missing the St Mary's defeat because of a family issue.
Defenders Matija Nastasic and Kolo Toure also missed the trip to the south coast after late returns from international duty.
Mancini branded the fixture demands caused by the internationals "ridiculous" but he spared his defence - which included midfielder Javi Garcia playing as a makeshift centre-back - from criticism.
"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," he said.
"Nastasic came back Friday and Kolo came back three or four days ago - plus the last time Javi Garcia played there, he played really, really well.
"On Saturday we played well in defence, but the problem was that we didn't give the defenders any help. They were left to play alone, and we conceded two goals the like of which I have never seen in my life."