Manchester United won their third successive Premier League title last season but they've never done four in a row and there's at least three sides that will want to keep it that way.

Here we take a look at each of the 'big four' and assess where they need to spend if they are to launch serious title bids next season.

Manchester United

The Red Devils have been sigfinicantly weakened by the sale of Cristiano Ronaldo to Real Madrid and with Carlos Tevez also on his way, Sir Alex Ferguson has got a big rebuilding job on his hands this summer.

Armed with £80million of Real's cash his first job was to splash £16million on Wigan winger Antonio Valencia, who clearly is not on the same level as Ronaldo.

However, it is impossible to find like-for-like replacements for the best players in the world so rather than start a pointless search to find the new Ronaldo, Ferguson must find players to fit into a new system - and the capture of Valencia suggests United may revert to a traditional 4-4-2.

That would allow Wayne Rooney to play in his more effective central role alongside Dimitar Berbatov but United are seriously going to miss the goals of Ronaldo, even if Danny Wellbeck steps up to the plate as Ferguson has predicted.

Karim Benzema remains on the Scot's wishlist but the young Frenchman seems an expensive gamble given his less-than-earth-shattering 17 in 36 in an average Ligue 1 for Lyon last season. He is undoubtedly a good player, but can United afford to risk £38million on potential?

The Stretford End would be licking their lips if Sergio Aguero was brought in but he commands an even bigger fee - £50million - and it remains to be seen whether Ferguson is willing to spend that sort of money.

But he looks desperately short of a striker now Tevez has gone and is a brave man if he relies on Wellbeck to produce the goods this season.

Owen Hargreaves will be like a new signing in midfield if he returns the same player but Ryan Giggs and Paul Scholes cannot be too heavily relied on again so further money needs to be spent on new midfield options unless Zoran Tosic is seen as ready for regular action.

United can at least stick by their back five for now, although they are reliant on Rafael continuing his progress and Ben Foster putting genuine pressure on Edwin van der Sar for the number one jersey.

Liverpool

Liverpool went close to the title last season but close isn't good enough for a club of their size and the Anfield faithful will be praying Rafa Benitez is adding, and not losing, players this summer.

The Spaniard won't have the sort of budget United and Chelsea are working with but he needs the funds for one more big signing - Glen Johnson has already been captured for £17million - to stand a chance of overhauling United.

They have the best goalkeeper in the league, three good options in central defence and now a right-back that will offer them more going forward than ever before, although his defensive frailties must be mentioned.

If Emiliano Insua fulfils his potential they will have the same talent on the left so Benitez can feel happy with his options in his own half, but there is work to be done further up the field.

Keeping hold of Xabi Alonso and Javier Mascherano is arguably the most important task of the summer, but how Benitez would love the money to grab the terrific David Silva from Valencia. He can play on both wings and behind the striker, areas in which the Reds need more depth, and would work wonderfully in tandem with Steven Gerrard and Fernando Torres.

Dirk Kuyt, Albert Riera and Yossi Benayoun are three reasonable options to play wide and the Gerrard/Torres partnership is as good as it gets, but David Ngog isn't good enough as the third-choice forward and Ryan Babel is too incosistent either on the wing or up front.

The bottom line is that Liverpool need a better quality of reserve and simply will not win the league unless they make one more big-name signing so Benitez must be given the money to snap up Silva otherwise the Reds will fall just short again.

Chelsea

Chelsea have endured a pretty turbulent time since Jose Mourinho left the club and Carlo Ancelotti has got a big job on his hands to turn things around at Stamford Bridge.

Guus Hiddink oversaw a big improvement in the second half of last season, helping Didier Drogba and Florent Malouda among others find their best form, but Ancelotti is chasing more than one piece to complete his jigsaw this summer.

The Blues need someone to challenge Petr Cech for his spot and have bizarrely targeted Ross Turnbull, but there is plenty of work to be done ahead of the six-yard box. An experienced centre-half is desperately needed to replace Ricardo Carvalho if he moves on, while their pursuit of Glen Johnson suggests Jose Bosingwa is either on his way or not fully trusted as a defender.

Branislav Ivanovic, Paulo Ferreira and Juliano Belletti are not good enough to hold down places and it would be one hell of a risk to rely on Michael Mancienne on a regular basis so Ancelotti has got plenty of work to do in defence alone.

Deco's departure should not hurt the Blues too much with Frank Lampard, Michael Essien, Jon Obi Mikel and Michael Ballack all good options in midfield but a new winger is needed to challenge the injury-prone Joe Cole, inconsistent Solomon Kalou and Malouda, who has at least found his form at last.

Yuri Zhirkov is reportedly on his way from CSKA Moscow and signing the Russian, who can play on the wing or at full-back, would possibly give Ancelotti enough wide options but he needs to spend in attack too as Daniel Sturridge is not likely to be challenging Drogba or Nicolas Anelka for a spot just yet.

Aguero, also linked with Manchester United, has been linked but it remains to be seen whether Roman Abramovic is willing to splash out £50million on one player these days.

However it cannot be doubted that big money needs to be spent if Chelsea are to challenge next season.

Arsenal

Arsenal finished a massive 18 points behind United last season and, although they were unlucky with injuries, Arsene Wenger needs to break the habit of a lifetime and buy some established players if his side are to stand a shot at glory.

Thankfully he has already spent big in one key area - spending a reported £10million to land Belgian centre-half Thomas Vermaelen. His young age and lack of height - he is only 23 and under 6ft - may worry some Gunners fans, but Vermaelen developed a reputation as a tough-tackling, dominating defender at Ajax and could be exactly what Wenger has been missing.

Denilson and Alexandre Song impressed in parts last season but the two 21-year-olds still lack experience and the midfield still lacks a really dominant Patrick Vieira-type figure alongside Cesc Fabregas. The Gunners need an imposing midfielder who can drag them forward when the going is tough and their passing game isn't working.

A failure to sign one will unboubtedly lead

to some more unnecessary defeats against more physical outfits - as it did at Stoke last season.

Wenger may also need to strengthen up front if Emmanuel Adebayor leaves as question marks remain over Nicklas Bendtner's ability as a first-team regular, although he has shown promise.

Robin van Persie and Eduardo are both capable players but the Gunners must find another physical forward if Adebayor does indeed move on and then cross their fingers for a season of few injuries and a genuine title challenge.