The recent exposure of Chelsea’s threadbare squad has led most to believe that Jose Mourinho will need to delve into the transfer market this month. However, past experience has shown that this will not guarantee success for his club, with many of Mourinho's previous signings not living up to their price tags. The most important players in his back-to-back league successes were all at the club before his arrival - Petr Cech, John Terry, Claud Makelele and Frank Lampard.

Although a few of the Portuguese manager’s signings have been relatively successful, most have not impressed and certainly not justified their price tag - either failing to fit into Mourinho’s rigid system or simply lacking the requisite ability to play in the Premiership.

It may actually be a testament to his ability as a manager that he has so completely dominated Arsenal, Liverpool and Manchester United the last two seasons despite so many key transfer busts. But let’s take them one by one:

Paulo Ferreira - Mourinho’s first signing as Chelsea manager. Won the Champions League with Porto and cost £13.2m but hasn’t really cemented a place at Chelsea. He has struggled to get into the team this season despite being the only real right-back at the club.

Mateja Kezman - Had a great goalscoring record at PSV and looked to be a great deal at £5m, but had one very disappointing season - a goal in the Carling Cup final aside - before being sold to Atletico Madrid.

Didier Drogba - Mourinho’s best signing. At first he looked unlikely to live up to his £24m transfer fee, but he has proved himself to be one of the best strikers in the world and the spearhead of Chelsea’s attack. Has already scored 19 goals this season, including five in Europe.

Tiago - A good midfielder who struggled to settle due to Mourinho’s squad rotation. Signed for £10m from Benfica and showed what he was capable of with this goal against Manchester United. http://www.youtube.com/watch?v=wdi4_CqL_8s. He was sold to Lyon for a loss of £3.5m a year later - where he has developed into the player Mourinho hoped he would be for Chelsea.

Ricardo Carvalho - Another Champions League winner at Porto, he was signed for £19.85 million. Although he has established himself in the first team, Carvalho has never really been convincing - an extremely clumsy player who concedes a lot of fouls.

Jiri Jarosik - A big failure at Chelsea. Signed from CSKA Moscow for around £5m and sold 18 months later for £2m to Celtic. Spent a year of this time on loan at Birmingham - who were relegated.

Asier Del Horno - Was very highly rated when Chelsea signed him for £8m from Athletic Bilbao and his aerial ability and height seemed certain to make him a success. However, he was injury prone and couldn’t settle in the team. Signed for Valencia for around £5m a year later, where he has yet to make an appearance due to injury.

Shaun Wright-Phillips - Has seen his reputation plummet since his £21m move from Manchester City. Hasn’t been able to hold down a first-team berth and it cost him a place at the World Cup, with the in-form Aaron Lennon preferred to him. Likely to leave this month or in the summer.

Michael Essien - Joined for £24.4m from Lyon and is proving to be a good signing. An aggressive midfielder who is prone to making rash tackles but nonetheless is ideally suited to the fast pace of the Premiership. Has been used in Chelsea’s problem right-back position. Scored a wonder goal against Arsenal which could keep Chelsea in the title race.

Maniche - Another of Mourinho’s former Porto stars, signed on a six-month loan from Dynamo Moscow. He didn’t even play enough games to claim a Championship medal. Has now joined Atletico Madrid after a good World Cup.

Michael Ballack - One of the world’s greatest midfielders but has not been entirely successful so far. He has not scored as many goals as expected but has quickly established himself as a guaranteed starter.

Salomon Kalou - His arrival was overshadowed by the signing of Andriy Shevchenko, but he appears to be settling in well. His hard-working style makes it likely he will succeed in the Premiership. The 21 year-old bases his game on Thierry Henry, but time will tell whether he has the same impact as the Arsenal star.

Andriy Shevchenko - Chelsea’s £30m record signing from Milan was supposed to be the man to propel them to European success. Has underwhelmed since joining but there have been recent signs that he may be finding form. It is worth noting that his arrival has sparked Drogba’s purple patch in front of goal. However, he may not be suited to the frenetic pace of the Premiership - perhaps preferring the slower, more tactical Italian game.

Hilario - Has proved to be a good back-up following injuries to Cech and Carlo Cudicini. He may in fact have propelled himself above Cudicini in the pecking order.

John Obi Mikel - Sir Alex Ferguson’s greatest ever piece of business. The Nigerian was never properly a Manchester United player, yet Chelsea paid them £12m to end the transfer tug-of-war between the two clubs. Lyn Oslo received just £4m for the player. Mikel is only 19, but so far has not lived up to his wonderkid reputation. Picked up a red card against Reading and has already been in trouble with Mourinho as a result of his bad attitude (he has apparently turned up late for training three times). Mourinho has not ruled out the possibility of selling him in the current transfer window.

Khalid Boulahrouz - Nicknamed 'The Cannibal' but has been more like a sacrificial lamb since joining from Hamburg for around £8.5m. Robbie Keane embarassed him at White Hart Lane and he has been swiftly dropped by Mourinho following a few high-profile blunders.

Ashley Cole - The long-awaited transfer saga finally ended on deadline day, but Arsene Wenger appears to have received the better deal with William Gallas adding more steel to the Arsenal defence. Cole has cemented a place in the team but he hasn’t shone as one of Chelsea’s better players so far this season. Will turn out to be good signing though - comedy two-man own goals aside.

Magnus Hedman - Tempted out of retirement to act as cover for the injured Cech and Cudicini. Hasn’t made an appearance yet, and Chelsea fans must be thankful for that, because the Swede was famously error-prone during his time at Celtic.

With the January transfer window now having opened, Mourinho will have an endless supply of money to buy the players he needs to supplement his squad for their assault on four trophies. But it’s hardly a guarantee that the Chelsea boss will snap up the right men to see them take the Premiership title for the third year running.

A a couple more Boulahrouzes and the London club will officially have taken two steps back