A 3-1 victory over Aston Villa on Saturday ended the Midlands side's challenge for fourth spot and the winner of Wednesday's encounter at Eastlands will be the favourite to claim the prize of Champions League football.
Mancini said: "It's going to be a final. We wanted one final this year and we have it.
"Now we must win against Tottenham, we have to play like we did today, we must be strong.
"It's maybe the most important game in the last year and we must win."
Mancini's future at City has come under question but the Italian would not be drawn on how the result against Tottenham would affect his position.
He added: "Everyone asks me this but I don't know. It's not important for me, it's important that the squad win the next game and get the fourth position.
"I do this job because I enjoy being a manager and enjoy football. I like this job."
Carlos Tevez's penalty, Emmanuel Adebayor's smart finish and Craig Bellamy's last-minute strike gave City victory over a Villa side who had stunned the home fans by taking the lead through John Carew.
Tottenham are just a point ahead in fourth place with two games remaining for each club.
City were excellently marshalled by Patrick Vieira while Marton Fulop, the keeper signed on emergency loan from Sunderland, overcame initial nerves.
Mancini added: "For Fulop it was difficult. He was so nervous in the first half, but second half he was very, very good and I hope the next game it will be quiet.
"Patrick Vieira played a fantastic game - he deserved this and I want to say thank you to all the players. They continue to play football and this is important."
Tevez appeared to have a slight limp when he was replaced but Mancini said he was tired rather than injured.
"Perhaps he worked two times in one day!" added Mancini, in reference to the Argentinian complaining about the manager arranging double training sessions.
Villa manager Martin O'Neill insisted he would come back next season doubly determined to take the club into the Champions League.
O'Neill will sit down with Villa chairman Randy Lerner next week to analyse the season, but said he is proud of his squad.
He said: "The chairman is coming over for a few days, hopefully take in the last game of the season.
"I will sit down with him and discuss all aspects about the season. If it's like he was telling me a couple of weeks ago - he's been immensely proud of the team.
"I am really proud of the team too, I have just said to them we opened the campaign this year with a loss at home to Wigan Athletic and up until today we shared with two other teams the least number of defeats.
"It tells you a lot about the character of the team, particularly after what happened at Stamford Bridge [Chelsea won 7-1].
"I know there's a view held that because Manchester United, Arsenal, Chelsea and Liverpool have lost more games this season that they normally would, then it's a weaker league - I'm inclined to think it's the other way."
In terms of the match, O'Neill said the result swung on Carew hitting the bar at one end, with City then going up to the other to make it 2-1.
He said: "We certainly kept Manchester city at arms length in first half. We looked excellent and looked dangerous on the break.
"Just when we looked comfortable in the game, we gave away the penalty and the moment after that Carew hit the bar and I thought we were going to follow up and put it in the net, but 25 seconds later it was 2-1 to them. It's a big, pivotal moment in a game of such magnitude."