The Italian has made no secret of his disappointment at losing out on the signature of striker Robin van Persie, whose goals drove behind United's charge to a 20th title which they sealed on Monday with a hat-trick from the Dutchman against Aston Villa.
"We need to have our target in mind (for next season), you should move very quickly, we didn't do that last year, we moved so slow," Mancini told a news conference.
"We lost the players who were our targets to improve our team. When you have a target you should move quickly. When you start in the market you need to buy important players who can help you during the season."
With the huge bank balance of their Abu Dhabi owners, affording big-name players is not a problem although since they failed to lure the likes of Van Persie when they were Premier League champions, it can only be harder when they are not.
They could still win the FA Cup as they face Wigan Athletic in the final on May 11 but that will not mask the huge 16-point gap between them and United in the league.
Trimming that deficit to something more respectable is still a goal for the season and they will be working on it starting with Saturday's home match against West Ham United.
"I don't think the gap is correct. We have five games, we have time to recover six or seven points, this is possible," said Mancini, adding that David Silva, Sergio Aguero and James Milner were all fit after injuries.
Mancini, whose team have 68 points to United's 84, did not want to take anything away from his neighbours' success but also pointed out where it had gone wrong for his team.
"The crucial moment was when we played QPR, Liverpool, Southampton - in that moment we had big problem with our players injured, we lost key points," he said, referring to a spell from the end of January into early February where they picked up just two points from nine.
"United had more attitude (to cope with) this, they have more experience for this than us.
"We did some mistakes, we did everything to win this title, but they did better than us. We could score more goals, we were missing a lot of goals," he added.
"They deserve to win this title, many congratulations for Manchester United for what they did in this season."