The Italian's men were crowned champions last term but could only watch on as rivals Manchester United sealed the 2012-13 trophy with a 3-0 win over Aston Villa on Monday and the 48-year-old has blamed their failed title defence on a failure to secure their targets swiftly.
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"You should move very quickly. We didn't do this last year," Mancini told reporters.
"We moved so slowly. We lost the players that were our targets to improve our team.
"I think that when you build your team you need to start like in this case, in the end of this championship. We have time but not a lot of time. We should have our targets in mind."
The Italian also admitted that United earned their league title this year by showing more "attitude" than his side but feels that City were unlucky in suffering "big problems" with injuries.
"United have more attitude than us, as a club, as a team," Mancini added. "We had big problems with players injured.
"They did better than us, they won a lot of games in a row, it was really difficult."