Chelsea's focus in the 2012 summer transfer window was on assembling a glut of attacking midfield talent. The acquisitions of Oscar and Eden Hazard to compliment Juan Mata in the midfield have proved inspired, and Chelsea are on course to beat last season's tally of 65 goals in the Premier League.
The problems they have encountered this season have mainly come from their defence, as they have dropped points from a winning position on six occasions in the Premier League, as well as having to come from behind to win four league games. Cesar Azpilicueta has settled in well at right-back and Ashley Cole has signed an extension to see him stay as the club's first-choice left-back for at least another year, but with David 'liability' Luiz moving to a more advanced midfield role and John Terry suffering several injury set-backs, Chelsea need to find some cover at centre-half.
This season's back four has been a far cry from the title-winning defence which conceded 15 goals in an entire campaign in 2004-05, and Roman Abramovich will need to bolster the back four in the summer if he wants Chelsea to start challenging for the League in 2013-14.
He could do a lot worse than to enquire about these players.
The 24-year-old German international is part of the central defensive pairing which saw Borussia Dortmund win the German domestic double in the 2011-12 season. His all-round game is excellent as he is strong in tackles and he can read the game at a level beyond his years, allowing him to make vital interceptions. Having signed a contract extension at Dortmund which runs until 2017 he would not be cheap, but he could represent the extra defensive dimension that Chelsea need.
Subotic is the other half of Borussia Dortmund's central defensive pairing as well as playing for the Serbian national team. He tends to stay back and clear up while Hummels goes forward to make tackles and they make a formidable pairing. Chelsea made an unsuccessful bid of £17.5 million for Subotic back in January 2011 and with his contract running until 2016 they will need to offer a significant sum to unite the 24-year-old with his compatriot Branislav Ivanovic.
Davide Santon moved to Newcastle United in 2011 having garnered much praise from Jose Mourinho who coached him at Inter Milan. The Italian's performances also prompted Marcello Lippi to compare him to the great full-back Paolo Maldini. He can play on either side of the defence and offers a great deal in terms of attacking support as well as cover for the first choice full-backs, although it is likely to take an offer in excess of £10 million to prise him away from Alan Pardew's Magpies.
The 31-year-old Manchester City defender is excellent at tackling and distribution, and has 10 years of Premier League experience behind him. Having served a six month suspension after failing a drugs test in 2011, he has not featured regularly for the Citizens and his contract expires at the end of the 2012-13 season. If he is available on a free transfer it could represent good value for Chelsea as they continue to develop youth team talents like Nathan Ake.
Any one of these players would add quality to the depth of Chelsea's squad and Abramovich will need to work hard to avoid any defensive disasters for next season.