The Manchester City manager needs it to rescue a season which promised so much but is rapidly turning into his worst nightmare.
Twelve points behind Manchester United in the Premier League. Out of the Champions League. Beaten 3-1 by Southampton last weekend. Not exactly what owner Sheikh Mansour would have had in mind after forking out more than £1billion since taking over City.
Winning the FA Cup, in which City face Leeds in the fifth round on Sunday, might not be enough to save Mancini's job considering this was the season the club were supposed to turn their attentions to ruling Europe, but it would provide a modicum of redemption.
But only if Mancini can coax some of Europe's highest-paid players to perform somewhere near their potential.
He needs captain and central defender Vincent Kompany to return from a calf injury to provide stability. He needs Yaya Toure to start providing the inspirational performances from midfield which were such a feature when winning the title last season.
He needs David Silva and Carlos Tevez and Edin Dzeko to score goals with the freedom they once did.
City, however, should beware. They face a Leeds side who have already dumped three Premier League sides out of domestic cup competitions this season.
Everton and Southampton in the League Cup and Tottenham in the FA Cup have all felt the bite of Neil Warnock's Championship side at Elland Road.
But this is a step up and this time they face a Premier League side who will be taking the cup seriously.
That is the recurring theme of this FA Cup weekend.
True, Arsenal manager Arsene Wenger might be tempted to rest one or two of his top players for Saturday's tie against Blackburn at the Emirates Stadium with a crucial Champions League match against Bayern Munich on Tuesday.
But the man who last lifted a major trophy in 2005 must ask himself the question: Do Arsenal have a better chance this season of winning the FA Cup or the Champions League?
The answer demands respect for the domestic trophy and Arsenal should be too strong for Blackburn.
We can also expect Chelsea to field a strong team when they take on Brentford in a fourth-round match at Stamford Bridge, despite Chelsea having played in the Europa League on Thursday night.
Chelsea manager Rafael Benitez would love to win the FA Cup. He would love to ram the most famous domestic cup trophy in the world down the throats of his relentless critics.
So where will the romance surface?
Huddersfield have a fighting chance when they entertain relegation-haunted Wigan and what a story it would be if Oldham could knock out another Merseyside giant in the shape of Everton to follow up their humiliation of Liverpool at Boundary Park in the last round.
The League One side have parted company with manager Paul Dickov since that thrilling 3-2 victory due to their slide into the relegation zone.
Such are the harsh realities of modern football, but Oldham, under caretaker-boss Tony Philliskirk, did show signs of rejuvenation in the league with victory in their last match over MK Dons, who have a fair chance of getting through to the quarter-finals themselves when they take on Barnsley at home.
It is difficult to see anything other than Manchester United progressing against Reading on Monday while the one non-league team left in the competition, Luton Town of the Blue Square Bet Premier, have the chance of glory against Millwall plus a heavy responsibility.
Luton v Millwall immediately conjures up images of the riot at Kenilworth Road in 1985, when a quarter-final tie between the teams saw 47 people injured, including 33 police officers, leading to a ban on away fans by Luton lasting four years.
The tie is live on television and the fans of both sides must prove Eighties' hooliganism truly has been consigned to history.
It is another reason the FA Cup matters this weekend.
Saturday's FA Cup Fixtures
Luton Town vs Millwall
Arsenal vs Blackburn Rovers
MK Dons vs Barnsley
Oldham Athletic vs Everton
Sunday's FA Cup Fixtures
Chelsea vs Brentford
Man City vs Leeds
Huddersfield vs Wigan
Monday's FA Cup Fixture
Manchester United vs Reading