Mancini's men were crowned champions last term but could only watch on as rival Manchester United sealed the 2012-13 trophy with a 3-0 win over Aston Villa on Monday, and the he has blamed the failed title defense on a failure to secure transfer targets swiftly.
"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."
Mancini 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 said. "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."