Mancini admits the players may not be happy to kick their heels on the bench - for example Edin Dzeko did not play a minute of City's 3-0 win over Wigan at the weekend despite scoring four goals in the previous match.
Mancini said: "They may not be happy - that's impossible because every player wants to play always, but I'm the manager and I need to take this decision.
"Now we have a difficult match coming up, we play every three days and these players like Edin, Kolarov, Savic, Nasri have played two games in four days and they need to recover.
"We have a long year and I need all the players to be available as much as possible and sometimes it's better that they rest.
"We have the first game in the Champions League for the club, it will be an exciting night but we need to have a focus and make sure we beat them."