England duo Gareth Barry and Shaun Wright-Phillips were given an extended break after the tournament in South Africa and the same applied to Carlos Tevez, Kolo Toure and summer signings, Yaya Toure and David Silva.
Barry and Silva played only part of the Europa League play-off first leg against Timosoara on Thursday, which City won 1-0 and Mancini said: "Those who played in the first half like Gareth and David played well.
"But I think all the players who played in the World Cup need time (to build up their fitness)."
He added: "We also need time for the players to get to know each other and to improve as a team."
Next up for City are Liverpool at Eastlands on Monday and Mancini is hoping Mario Balotelli will be fit for the match.
The former Inter Milan striker took a kick on the knee in Romania after scoring the winner on his debut.
Balotelli, 20, looked the part but Mancini insists the player is on a learning curve in England.
"He is a good player, even though he is young," added the City boss. "But he needs to know about the Premier League."
Source: PA
Source: PA