Sportsmail can reveal the United manager is also sweating on the fitness of Patrice Evra, one of his most valuable defenders.
The Frenchman is struggling with a thigh injury suffered against Schalke on Wednesday, and Ferguson is nervous about risking him when aggravating the problem could rule out left back Evra for the Champions League final on May 28. 'It is a big call,' said an insider.
Doubt: Patrice Evra (left) faces a race against time to be fit for Sunday
Ferguson employed familiar tactics by focusing attention on the match officials.
Asked if a poor decision was a concern, he said: 'That's definitely our big fear, to be honest with you. Howard Webb is the best referee in the country, but we hope it's our turn for a little bit of luck.'
Ferguson's concerns are fuelled by controversial incidents in his side's last two Barclays Premier League matches against Carlo Ancelotti's team.
At Old Trafford last season, Didier Drogba scored Chelsea's decisive second goal from an offside position while, in March at Stamford Bridge, Ferguson felt David Luiz should have been sent off before scoring the equaliser in a match United lost 2-1. Ferguson received a touchline ban for then questioning referee Martin Atkinson's integrity.
In control: Ferguson hopes Howard Webb (second right) will have a good game
United did get lucky in their Champions League quarter-final first leg against Chelsea last month, surviving claims for a clear penalty when Evra fouled Ramires.
But,for all the confidence Ferguson has in Webb, the referee for last summer's World Cup and Champions League finals, he still took the chanceto apply some pressure.
Ferguson insisted Wayne Rooney was OK to play, having missed the game against Schalke on Wednesday because of a slight hamstring problem.
Hot water: Sir Alex Ferguson was hit with a ban following his comments about Martin Atkinson at Stamford Bridge
He said: 'Last Sunday's defeat at Arsenal has made us aware of how important the game is this week. It should be a fantastic game, and if we win it we should win the league.'
One man who will not be happy to see Rooney playing is Ancelotti, who yesterday said he would rather have the United striker in his team than Lionel Messi.
Ancelotti also believes United can cause an upset and beat Barcelona in the Champions League final.
'Barcelona are favourites but United are faster, more intelligent and have the skill to win,' said the Chelsea boss.
Better than Messi? Rooney will be closely watched by Chelsea
'Everyone goes for Messi but I would take Rooney. Messi is out of this world and maybe he is reaching the level of Maradona.Rooney, however, is closer to the reality of football.He puts his talent out to the rest of the team and this explains the difference between players and outstanding players.'
But Ancelotti claimed he would not man-mark the United star, saying: 'It means changing our philosophy too much.
'We want to play our football, not just control their skills and ability. Rooney is the key player for United, and the last time against us Ryan Giggs was the best player. He passed three balls and they scored three goals.'
Pressure: Webb has officiated the World Cup final and Champions League final
The Chelsea boss praised his players for getting themselves in a position to challenge for the trophy.
He said: 'Nobody thought, two months ago, that we'd be able to do this. This team are still alive and that means the players have very good character, personality and are very professional.
'If we are able to win at United, it is not over. Goal difference will be important.
'We have difficult games against Newcastle and Everton, so nothing is decided, but it would be important to top the table. It would be after a long time. So it's time to win.'
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Explore more:People: Alex Ferguson, Ryan Giggs, Didier Drogba, Wayne Rooney, Patrice Evra, Lionel Messi, Carlo Ancelotti, Martin Atkinson Places: Newcastle, Barcelona