Mancini: Man City's missed signings cost title chance
"We would probably be at the top... probably also in a Champions League quarter-final," the City manager said.
"We had a chance to get three or four players that would have improved our team but now it is finished with."
And he added rivals Manchester United "continued to improve their team".
All five top targets identified by Mancini last summer opted against joining City.
Those at the top of the Italian's list were understood to include Netherlands striker Robin van Persie, who instead joined Premier League leaders United from Arsenal, and Belgium's Eden Hazard, who signed for Chelsea from French side Lille.
Additionally, Athletic Bilbao's Javi Martinez chose Bayern Munich, Daniel De Rossi stayed in Italy with Roma and City refused to meet Liverpool's £27m valuation of Danish centre-half Daniel Agger.
"We had the chance to get important players, players that this year could have been the difference in this team," continued Mancini. "But it is useless to talk about this now."
City go into Monday's Manchester derby at Old Trafford trailing rivals United by 15 points. With just eight games remaining, Mancini has already conceded the title and the Italian believes that, despite City securing their first title in 44 years last May, United's aura remains unchallenged.
"No one player plays very well against United because they play with fear," he said.
"Because United have an importance as a team and a club, every team that plays against United plays very softly because they think 'this game is difficult, we cannot beat them'.
"I don't think that other teams don't play 100% but their mentality is poor in that game because they don't think they can do anything. But that is not true if you go there and play with a good mentality. In football, you can beat every team."
Mancini, in his third full season at City, concedes that United's success over an extended period means it will be a long time before the blue half of Manchester can challenge the prestige and history enjoyed by their rivals.
"We are young as a team," he said.
"We don't have the same importance as United in this moment. Maybe in 10 years we will be the same. This is history, and the history is very important.
"We need to improve as a team and as a club but for this we need more years, we can't do this in two years. I think in 10 years, if we improve, if we win the league every year, it will be the same for us.
"You can't win every year. Last year United didn't win anything, apart from the Charity Shield - but they continued to improve their team."
Monday's meeting will be the first time City have been back to Old Trafford since last season's 6-1 victory, a result that prompted many to suggest the balance of power in Manchester had shifted for good.
But although Mancini is proud of that result, he is not predicting a repeat on Monday - and accepts that United will be worthy champions.
"Six-one is in history," he said. "We can talk about this for another 200 years, but it is impossible to do this another time.
"We know we can beat them [on the day] because they are not better than us. We need to go there thinking we are like them, that we are a strong team like them.
"But they deserve to win this title."
