City relinquished their Premier League crown to local rivals Manchester United with four games left to play of the campaign and the Italian believes the lack of a consistent goalscorer was key in their slipping behind.
Edin Dzeko has been the team's most potent attacker, with 12 goals in the league, while Carlos Tevez and Sergio Aguero trail just behind on 11 - January departure Mario Balotelli notched just once - and Mancini says that he will be looking to bolster his squad during the summer.
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"Last year we scored, I think, 20 more goals by this stage and we are missing 20 and that is too many. I think that to score another 10 to 12 goals would have been enough to fight for the title.
"We can't know which player can score 25 goals next year. If you know this, you are a magician but I think we have good players; we were unlucky this year, probably.
"It was an unlucky season. Now it is important that we win the FA Cup and after we have time to buy to improve our team."
Mancini was speaking after seeing City held to a 0-0 draw by Swansea City, with Dzeko in particular unable to convert when in a good position.
"In the first half, we missed three incredible chances - one [Samir] Nasri, two [David] Silva," he reflected. "In the second half, there were another three.
"This is our season, exactly, in 90 minutes but, in the end, it is one point. It is not good but it is enough."