The Italian guided City to their first top flight title since 1968 last season and is well aware that the club could face difficulties in trying to defend that triumph in such a cut-throat competition.
"We did a good job for the last two years and I hope we can do a better one in the next five," Mancini told City's official website.
"But we cannot rest or start thinking that because we won the title that we are the best.
"It is more difficult to win the title when you are defending champions. We start again as if from nothing, we must forget that we won the title. Every team will want to beat us and we must work harder than ever.
"I think there are four or five teams that can challenge but now, before the transfer window is closed, I think (Manchester) United and Chelsea will challenge us for the title."