Mancini insists City's additions made no difference compared to Manchester United's arrivals.
"They bought Van Persie, who has scored important goals. That's why they've won this title, not because they've played better than us.
"United are a good team but Van Persie has made a difference. They have good players, they have Rooney, Nani, Valencia, six or seven good strikers, and they've bought Van Persie who has scored 23 goals in total for them.
"Last year, in the end we won on goal difference. We deserved to win the title but we won on goal difference.
"But if we have the same team and instead you put Van Persie and Shinji Kagawa in United's team...We made a mistake. It can happen. We can't be perfect."