Chelsea's lead over United was trimmed to nine points after they were held to a goalless draw by Manchester City last Sunday and Ferguson is unconvinced by the form of Jose Mourinho's team.

"We are coming to the part of the season where you want to have good form," Ferguson told reporters on Friday.

"I don't think Chelsea have been playing as well as they have been playing and it could make it interesting to the end of the season."

Chelsea face a difficult game at Champions League hopefuls Everton on Saturday, a day before United face a derby game at Manchester City.

"We've got to maintain our form and also hope that we get some help with Chelsea dropping points," added Ferguson, whose side still have to play Chelsea at Old Trafford before the end of the season.

"The thing about the premier league is that it doesn't matter who you play when you get into the run-in, every one is a difficult game."

United could be without winger Cristiano Ronaldo [Images], who sustained an Achilles tendon injury following a challenge by Chelsea's Damien Duff during the friendly between Portugal and Ireland on Wednesday.

"Cristiano Ronaldo had a bad tackle on Wednesday and he is doubtful for Sunday. It was a tackle from young Duff. I was surprised because he's not that type of player, but things happen in games, you never know," said Ferguson.

United have lost their last two games at the City of Manchester Stadium but meet their mid-table local rivals unbeaten in 16 league games.

"If you are going into a derby game you want to go into it in good form with good confidence and at the moment we've got that," Ferguson added.

Central defender Mikael Silvestre misses the game through suspension, but Rio Ferdinand and Ryan Giggs will be fit after pulling out of internationals because of injury.

Strikers Ruud van Nistelrooy [Images] and Alan Smith are still recovering from injuries but both are due to return to training next week.

"Both are back and it's terrific to have them," Ferguson said.

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