The 70-year-old thinks the huge disappointment of defeat would have an adverse affect on their rivals, but did praise the way Mancini has progressed with the team this season
Manchester City head into their home game with relegation candidates QPR needing a win to clinch the title, provided the Red Devils do not achieve an unlikely nine-goal swing.
The Citizens are firm favourites to triumph considering they have won all but one of their league fixtures at the Etihad this season, with Mark Hughes' men also holders of the worst away record in the table.
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He told Sky Sports: "City have got to win, but it's an enormous challenge for them, because the disappointment of losing the game would be unbelievable.
"It's untold at this moment in time what effect it could have on them."
Sir Alex then hailed his opposite number's role in City's title challenge, insisting that despite having huge sums of money to spend, moulding players together into a championship winning team is a tough task.
"To win the league is not easy," the Scot continued. "Many people underestimate how difficult it is to win the league.
"Of course, Roberto Mancini has had a big financial advantage over everyone, but you have still got to put a team on the pitch and pick the right players.
"The manager must take the most credit if you win the league over here, because when you have squads of players, trying to orchestrate the harmony in the place is very important.
"Making everyone feel they have played their part is not easy in the modern game. To win the league is a massive step forward for Mancini."
The 2011-12 season will no doubt go down as one of the most exciting ever, with various different battles still to be decided on the final day.
Comparisons have been drawn with the duel between Newcastle and United in 1996 when Kevin Keegan's Magpies looked to be heading for the trophy before Sir Alex's men hit back to claim top spot.
However, the former Aberdeen manager was quick to point out differences between Keegan and Mancini, claiming that the former may have been too focused on entertaining.
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"Kevin was different," he added. "He was a romantic. He brought in so many attractive attacking forwards, [Les] Ferdinand, [Peter] Beardsley, [Faustino] Asprilla, [David] Ginola, [Philippe] Albert.
"It was a fantastic array of players. They were entertainers, but that was Kevin. He wanted to win at Newcastle with the most exciting team of all time.
"I always remember when he went to Belgium and won 5-0 away from home in a European tie [at Royal Antwerp in 1994], he said afterwards, 'We are invincible'. Dearie me.
"That was Kevin. He was such a romantic for the attacking part that Newcastle had at the time."
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