The Italian has urged his club to splash the clash in the transfer window if they are to remain competitive in the Premier League and keep up with their title rivals
The reigning champions are currently 15 points behind league leaders Manchester United ahead of Monday's derby and the Italian has stressed that money must be spent if things are to be different next year.
Mancini told The Mirror: "I don’t know how much money we can spend in the summer, but in the past United spent a lot of money to buy important players and now they don’t need to spend £100 million every year.
"United are a top team. Now they just need to buy one top player every year because they are strong.
"For us it is different. We need to do it this way now and, maybe in the next three or four years after that, we would only need to buy one player every summer because we have a strong team.
"I repeat that we are not like United. They have a big history and we need a lot of years to compete with this.
"We need to improve our team every year and this summer we need to buy the players who can make the difference, important players that can improve our team and make us better."
Mancini accepted that City made mistakes in the transfer market last summer and pointed to United’s acquisition of Robin van Persie from Arsenal as pivotal in this year’s title race.
"United bought Van Persie, who has scored important goals. That’s why they’ve won this title, not because they’ve played better than us," he added.
"United are a good team, but Van Persie has made the difference. Last year, in the end we won the title on goal difference.
"We deserved to win the title, but if we have the same team and instead you put Van Persie in their team, that has made the difference.
"We made mistakes. It can happen. We can’t be perfect. It is important we understand this."