Mancini believes that his team have been short of the big goals required to win key matches to maintain an effective title challenge.
He told The Mirror: "Last year we played really well because we scored a lot of goals. We were the best defence, like this year, like two years ago.
"But this year the only difference is the goals we didn't score. If we'd scored maybe 15 goals more, probably we would be on the top. We've been missing important goals in games we deserved to win easy, West Ham, Chelsea away, Sunderland, Arsenal at home.
"We lost a lot of points because we didn't score enough goals. This is the difference. We also had a lot of injuries."
Mancini is also adamant that the club must spend heavily this summer to compete at the highest level.