By Pete Oliver

MANCHESTER (Reuters) - Manchester United manager Alex Ferguson said on Friday his Premier League leaders would not underestimate Arsenal when they face their old foes at Old Trafford on Sunday.

United have enjoyed their best start to a Premier League season with four wins from four, while a mis-firing Arsenal lie 17th after taking just two points from their three games.

"I don't think we can under-estimate Arsenal," Ferguson told reporters. "We will need to play to our best capabilities, but I am encouraged with the atmosphere around the place, and the team spirit."

Ferguson believes that Arsenal, who finished fourth in the league last season, are feeling the effects of losing some of their key players.

"Maybe there is a transitional period at Arsenal at the moment because they have lost big figures like (Patrick) Vieira, (Dennis) Bergkamp, (Robert) Pires and Ashley Cole.

"Just like us, we lost big players and you have to be patient for the next stage of development."

United's clashes with Arsenal have been tempestuous affairs in past years.

Although some of the sting has been removed by the departures of midfield adversaries Roy Keane and Vieira from their respective clubs, Ferguson still expects a demanding time on Sunday.

"They are always going to be competitive games," he said.

"I don't think you can avoid that, with two great clubs with the history they have, who have been competing against each other for the titles for years. What you want is to make sure that the game is done fairly."

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Victory would put United 13 points clear of the London club, though Ferguson is trying to concentrate more on his team's performance than opening up such a large gap.

"We had a similar situation two years ago when they came with their unbeaten record and if we had lost that we would have been 13 points behind, so we are not looking into that deeply.

"We are interested in the situation regarding our form and to continue that we need a good win on Sunday," he said.

As if the game needed extra spice, it comes on a day when champions Chelsea host Liverpool in another showcase match involving last season's top four sides in the final table.

"It's so early in the season for the top four to be meeting each other. It has got an extra significance," he said.

Ferguson can count on past Arsenal tormentor Wayne Rooney, who returned from suspension for United in their 3-2 midweek win over Celtic and who had missed the end of last season with a broken foot.

"Over the last four and a half months he has played one game for England and was then sent off against Portugal after an hour," Ferguson said.

"He played 35 minutes against Porto (in pre-season) and one game against Fulham so he is not any different from anyone else who plays football. They need games and he will be far better on Sunday."

United will be without the injured pair of forward Ryan Giggs and midfielder Park Ji-sung but winger Cristiano Ronaldo returns after missing the game against Celtic through suspension and is expected to start.