Saturday's 3-1 victory over Aston Villa 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.
"It's going to be a final. We wanted one final this year and we have it," said Mancini after Saturday's win.
"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.
Source: PA
Source: PA