By ESPNSTAR.com staff
Chelsea have only dropped two points so far but have the sternest test against league runner-ups Manchester United on Sunday.
Rooney knows that it won't be easy for United to come away with a result given the Blues' red-hot form and he feels that Roberto Di Matteo's side have changed remarkably since winning the Champions League back in May.
Buoyed by the success, Chelsea owner Roman Abramovich also loosened the purse-strings allowing their Italian manager to sign the likes of Oscar, Eden Hazard, Cesar Azpilicueta and Victor Moses.
And the England international feels that the two factors could combine to prevent his side from catching up on the league leaders although he believes that a strong defensive showing could lay the foundation for a first win at Stamford Bridge since 2002.
"They have started brilliantly. It's clear winning the Champions League last season has given them a lot more confidence," he told the Guardian.
"They have brought in some fantastic players and added more speed to the team.
"The important thing first of all is to make sure we defend well against them and then try and score.
"We have to make sure we are at our best to try and beat them."
The forward, who has also operated as a midfielder to accomodate Robin Van Persie this season, is also wary of Fernando Torres despite the Spaniard's inconsistent showings of late. The Chelsea striker has scored six goals from 13 outings but was dismal in the mid-week game against Shakhtar in the Champions League.
"Torres is still a threat, of course. I don't think he gets the credit he deserves because he works hard for the team.
"Sometimes the goals go in for him other times they don't but he's always trying."
United needed a comeback to beat Portuguese side Braga in their Champions League game over the week but Rooney understands Ryan Giggs' recent call to his team-mates to shore up the defence and prevent an early deficit.
Although United have been lucky recently with comebacks having become something of a regularlity, Rooney knows that Chelsea may not be as lackadaisical an opponent as the others that they have faced thus far.
"We can't afford to do that [let in early goals] this time.
"It happened against Braga and we did well to get back in the game and win it. But you can't afford to be going one or two goals down against Chelsea away."
The Red Devils will be up against Chelsea once more in the Capital One Cup on Wednesday.