City cut the gap to leaders Manchester United to five points with as many games remaining as they eased to a 4-0 win over West Brom, while their rivals were shocked at Wigan.
But Mancini feels it may be too little too late, and said: "United are a fantastic team and I don't think they can lose five points. For us it is important that we finish this good season for us well. This is the best season since 1968 and this is important."
He added: "I fight always, every day, and also my team but now I think it is too late.
"I think they have fantastic spirit, United. We don't have the same spirit and for this reason I think this is very difficult."
West Brom offered little resistance to City and manager Roy Hodgson had no complaints about the result.
Hodgson said: "I thought we regrouped and kept ourselves in the game in the first half and worked hard to restrict the chances.
"But we always knew we could not give away an early second goal because they were playing very well and we were finding it hard to get a foothold in the game. After the second goal we conceded a third one very quickly.
"After that we were looking at 30 minutes left to play against a very confident and very skilful Manchester City team.
"We had to keep going, which we did and I can't criticise my players for that."
Source: PA
Source: PA