Michael Essien's latest injury is a dreadful blow to the player that is arguably the best defensive midfielder in the English Premier League and European football.
Sustaining a long-term injury at any time is a terrible misfortune for a player, but the timing of Essien's latest knee problem could not be worse.
With newly appointed Chelsea boss Andre Villas-Boas looking to establish a new system and setup for the Blues, it would have been beneficial for Essien to have been there from its off. If Villas-Boas's new-look Chelsea get off to a flying start, he may well be loathed to tinker with it.
Essien's latest injury is a rupture of the interior cruciate ligament and meniscus, and, although successfully operated on, will leave the midfielder out for six months.
Although Chelsea have not delved into the transfer market to find a suitable replacement for Essien, here are 12 players that should be in their sights if they decide to do so.
Yes, Jon Obi Mikel is already a Chelsea player and, therefore, they would not need to delve into the market to buy him.
But a special mention must go to Mikel; he would be the ideal replacement for Michael Essien.
Mikel has played in the defensive-midfield role for Chelsea and played it well. He has matured into an expert in the position and has been the natural replacement for Essien during the Ghanaian's previous absences.
Although not as formidable a force to be reckoned with as Essien, and not a much of a goal-scoring threat as the Ghanaian, Mikel has proved that he has the stamina, defensively qualities and steadiness required to play in Essien's role.
So an honourable mention must to go Mikel as a replacement for an again injured Essien.
The 30-year-old Inter Milan and Argentine international, Esteban Cambiasso, would be an ideal replacement for the injured Michael Essien.
Cambiasso is extremely experienced when it comes to top-flight football, having played 41 times for Real Madrid and over 200 times for his current club, Inter Milan. On top of this, he is no stranger to Champions League football, having played in and won the competition.
The Argentine is renowned for his tenacity, hard work and stamina. His work ethic is second to none and his technical ability is top drawer.
As a driving midfielder and a powerful presence in the middle of the park, Cambiasso has few rivals.
Certainly if Chelsea were to look to replace Essien, Cambiasso should be a top target.
The young Brazilian was signed by Tottenham Hotspurs in the summer of 2010, in a somewhat surprising transfer.
The 22-year-old has gained valuable Premier League experience and been a steadying figure in the midfield for Spurs.
Although not as experienced as Michael Essien, he seems to display similar characteristics and could well develop into as formidable a presence. Sandro is certainly thought very highly of by his fellow international teammates and great things are expected of him.
Sandro has a reputation of being incredibly fit. He's a true box-to-box midfielder with a ferocious tackle and he's an extremely hard worker.
Having played for Spurs in the Champions League last season, Sandro would not be phased by Chelsea's own Champion's League games next season.
Sandro may not be the finished product, but as a long-term placement for the injury-prone Essien, he may well be a player Chelsea should consider.
German Club Bayer Leverkusen's Chilean central midfielder, Arturo Vidal, has been earning rave reviews for his performances in the Bundesliga and internationally.
The 24-year-old has played for Leverkusen on 117 occasions and scored 15 goals, but it is the defensive side of his game that Chelsea would consider buying him for. However, last season saw him operate in a more advanced position and score 10 goals.
Vidal has been attracting the attention of the likes of Juventus and Arsenal during this transfer window and is the type of player that if asked to defend could do so effectively, and if required to play further upfield could do so also.
No doubt a talented player, Vidal may well fit the mould as a replacement for Michael Essien.
Real Madrid and Argentina central midfielder Fernando Gago has been linked with several clubs in recent weeks; if Chelsea are to look for a replacement for injured Michael Essien, then the Blues should also be one of them.
Gago's opportunities under manager Jose Mourinho have been limited—and injury has not helped his cause.
But the Argentine is a talented midfield presence and, when playing, shows good poise and that he is comfortable on the ball. As with many of his countrymen, Gago is not afraid to be aggressive in the tackle and is technically a very gifted player, with an eye for a good pass.
Having dropped too far down the pecking order at Real, Gago has come onto the radar of top clubs such as Liverpool.
Gago has been with Argentina at the Copa America, but with their exit from the tournament Gago will now look to find a new club, as he likely wants more first-team opportunities.
Athletic and skillful, Gago would fit into Essien's boots well, but he may need time to adapt; however, his tenacious Argentine spirit would hold him in good stead if Chelsea went in for him.
Coincidentally, both players wear the same number for their respective clubs: No. five.
Manchester City's Dutch midfield enforcer, Nigel de Jong, has the reputation of being one of the hardest and toughest central midfielders in English football.
De Jong's presence in City's midfield provides Roberto Mancini's more creative players the opportunity to flow forward, knowing that De Jong will be there to provide defensive cover if needed.
Having played in the European Championships and World Cup for Holland, De Jong is no stranger to the big-time and high-profile matches.
His goal-scoring abilities may not be the greatest, but his true value to his team is his robust presence in the middle of the pitch.
A first-team starter for City, Chelsea would certainly have to dig deep if they were to look into bringing the Dutch midfielder to Stamford Bridge.
As a defensive midfielder, De Jong would certainly match the physical presence that Michael Essien provides to Chelsea.
The 27-year-old Italian international, Daniele De Rossi, has been with only one club in his professional career. A product of the AS Roma youth system, De Rossi has appeared for the Roman club 357 times, scoring 43 goals.
This powerful, aggressive and highly technically gifted footballer has represented Italy at all levels, from Under 19s all the way to the full national level, where he has collected 63 caps.
As Roma's first-choice defensive midfielder, Chelsea may have a tough time luring him away from his boyhood club.
A player with great stamina and attitude to the game, De Rossi is known for his ability to break up attacks and surge forward or drive his team on.
Many of his best attributes are those that Chelsea see in their own defensive midfield star, Michael Essien.
Having forged his reputation as a midfield enforcer at Wigan Athletic, Honduran international Wilson Palacios made the move to a bigger club in January 2009 when he moved to Tottenham Hotspurs.
In his time at Wigan, Palacios demonstrated a commanding and powerful presence in midfield, with the ability to break up the play of the opposition and set up attacks for his own team.
Palacios's stamina is right up their with the best of them and he is often the engine room of the teams he has played for. At Spurs he is the solidity and dominating presence in midfield.
Not the most gifted of goal scorers, Palacios's ability lies in giving his more attack-minded teammates the freedom to play their best.
The Honduran could certainly match Michael Essien when it comes to crunching challenges, maybe even surpass him. His powerful surges from midfield with the ball are on par with Essien's, but his shooting, unfortunately, is not.
However, if an aggressive, ball winning, powerful presence is what Chelsea need, then a move for Palacios would not be a bad one.
In August 2010, Newcastle United signed Ivory Coast central midfielder Cheik Tiote—and from the get-go he has caught the eye.
In an impressive debut against Everton, the Ivorian has shown that he is far from out of his depth in the English Premier League.
Tiote has a never-give-up attitude, is calm in possession and doggedly looks to gain possession when not. His all-action, totally-committed displays have caught the eye of top European clubs.
In his first season at Newcastle and in the Premier League, Tiote has been the rock in the Magpies' midfield and has rarely put a foot wrong.
The Ivorian may not have the experience that others have, but if Chelsea are looking for a tenacious, hard-working central midfielder, then one of the upcoming talents of the Premier League, Tiote, should be on their list.
The hard man in Brazil's midfield is Juventus defensive-midfielder Felipe Melo, who was famously sent off for stamping on Holland's Arjen Robben in the last World Cup quarterfinals, ending any hope of Brazil's advancement in the tournament.
Melo is a tough-tackling, hard-working midfield enforcer in the Dunga mould and has a tendency to get himself in trouble at times on the pitch, with yellow and red cards aplenty.
But the Brazilian's commitment to his teams cannot be faulted; he gives 100 percent when on the field of play.
He's not the most prolific of goal scorers, but scoring is not what Melo is about. His game is all about winning possession back when his team does not have it, and driving forward with it when his team does.
Determination, desire and passion would be the hallmarks of Melo's game.
Where Michael Essien stepped into the shoes of Claude Makelele, the best defensive midfielder while playing, it was Lassana Diarra who took over from the legendary Frenchman on the national team.
If the Blues delve into the transfer market to look for a replacement, it would be a fitting coincidence if Diarra took over for Essien.
The comparisons with Makelele are there for all to see: He's tenacious in the tackle, irritatingly hard to shake off and has bags of fitness and energy.
Again like Makelele, Diarra does not have a good record for scoring goals, but his all-around defensive game is second to none.
With the arrival of more high-profile stars at Real, Diarra is not an automatic starter, but has still featured in a fair share of Real's matches.
Diarra would be a fitting replacement for Essien, but his lack of goals would be his handicap. However, if others can shoulder that burden, the Blues may finally have found a replacement for Makelele.
The current Football Writers' Association's Footballer of the Year, Scott Parker, is still currently on the books of relegated West Ham United and would welcome a return to the Premier League.
A former Chelsea player, Parker never really made a big impact at Stamford Bridge. But since his departure from the Bridge, the Englishman has gone from strength to strength, particularly at West Ham.
Parker has displayed great courage and leadership, driving West Ham on when all seemed lost. Despite not being able to help them avoid the drop, Parker has emerged as the Hammers' talisman.
There are similarities between his game and that of Michael Essien—both drive their team forward, are excellent in defense and yet have a keen eye for goal. Last season, Parker chipped in with five goals from his defensive-midfield position.
It would be a welcome return for Parker if he came back to Chelsea, as he has unfinished business there. Parker arrived from Charlton Athletic in January 2004 in a big-money transfer, but failed to break into the midfield, which at the time was dominated by Claude Makelele and Frank Lampard.
However, times are different now, as is Parker; he is now far more experienced and well established as England's top defensive midfielder.
He would be the perfect choice to take over for Michael Essien while he is away—and then to battle with for the starting spot when he returns.