While Rooney is expected to return later this month, the England international misses arguably United's most important fixture of the season as they look to move four points clear of their rivals with victory.
Related ArticlesCarlo Ancelotti: United still dangerousWayne Rooney suffers 'minor ligament damage'Wayne Rooney faces a month on the sidelinesTitle run-in: key clashesDean to referee United v ChelseaChelsea boosted as injuries hit rivalsThe United striker has scored 35 goals in all competitions, 26 of them in the Premier League, and his absence at Old Trafford has already given Chelsea a massive incentive and one Ancelotti is hoping his side can make the most of.
"Everyone knows how Wayne Rooney is important for this team, he is a fantastic player," said Ancelotti. "He has had a fantastic season but his injury could be an advantage for us.
"Maybe sometimes a team can give more than 100 per cent when an important player is out. We have to pay attention to this.
"We need to show our play and do it for 90 minutes and stay concentrated. Man Utd are a very dangerous team and have a lot of players who can create difficulties for us.
"But we want to keep control of the game and to do this in Old Trafford is not easy. But we will try.
"Saturday is a very important game because we play a fantastic team who are top of the league now but I have always said the title will be decided at the end of the season.
"We will have a very good chance because our moment is good. We are fit and our confidence has improved after wins against Portsmouth and Aston Villa.
"We want to do our best. The destiny after the game will be with the winners, but nothing will be decided.
"If we are able to win, it is better for us. A draw is not a bad result because there are a lot of games and everyone of them is very important.
"But it is very difficult to win at Old Trafford. It is a fantastic stadium for me also, not only for Sir Alex. I won the Champions League there in 2003 and that is the best memory for me in my career."
Ancelotti has been boosted by the return to fitness of Didier Drogba, who missed two days' training with a minor groin problem.
"Drogba was out but trained very good today," said Ancelotti. "We have trained very well this week. The team is fit.
"He is a very important player. He has scored 30 goals. The other strikers did a great job against Villa without him, but he is very important to us."