Manchester United boss Sir Alex Ferguson accepts they face a difficult task to win a fourth consecutive Premiership title this season.
He said: "Winning four titles is something we'd like to do and it would be a great achievement.
"We know how difficult it is to win this league. We accepted a good challenge last season in terms of having to go to Japan [for the Club World Cup] and then play catch-up and to win the league was a great achievement.
"We don't have that problem this season, although I wish we
did. But [trying to win four titles in a row] is still a great
challenge for us."
Ferguson added: "It's difficult to say [who the biggest threat is]. I think ourselves, Liverpool, Chelsea and Arsenal are looking at what's happening at Manchester City with great interest and whether it can work in terms of the money they've spent.
"They're on the back pages every week now and there's a lot of interest in their situation particularly as none of us have really gone into the market in a big way. We've sold players in fact - Liverpool sold Alonso, we've sold Ronaldo, Arsenal sold Adebayor ... the big four have actually been really quiet [in terms of buying].
"So we're all interested in how it's going to pan out at City."