England trio Gareth Barry, Shaun Wright-Phillips and Joe Hart are not due to join up with their team-mates until July 19, when the club will be in the United States.
Other City stars are also on official leave after their World Cup duty, while Nigel de Jong, Jerome Boateng and David Silva are all still involved out in South Africa.
But those who checked in on Monday now face a rigorous week of double training under the guidance of Mancini, assistant Brian Kidd and new first-team coach David Platt.
The squad that heads for a further week of pre-season training in Portland, Oregon, will even have a morning session before jetting to the States on Saturday afternoon.
Mancini said: “Yes, it is hard work - now is the important time because pre-season is most important for the whole season. It is important we work very well in these 14 days.
Double training sessions are normal when you prepare a team for the year
“This year will be long and very hard, because we play probably three games a week. We can work morning and afternoon, and it is important that we do that.
“After the World Cup the players have to have time to recover well, because they will be very tired. But for us it will be important to prepare very well this month, for sure.”
City’s first match of their pre-season schedule, against Portland Timbers at the University of Portland, will be on July 17. The last is at home to Valencia on August 7.
