Ancelotti occasionally flirted with formations in his two years at Chelsea (namely the 4-4-2 diamond and 4-3-2-1) but from 2004 to 2010, the Blues played relatively unchanged.
Claude Makelele or John Obi Mikel sat in front of the back four while Frank Lampard, Michael Essien, Deco or Michael Ballack played centrally.


When the manager was not under pressure to play Andriy Shevchenko, Didier Drogba led the attacking line with Damien Duff, Arjen Robben, Joe Cole, Florent Malouda, Nicolas Anelka or Salomon Kalou providing support on the wings.

It led to a Champions League final (2008), three Premier League crowns (2005, 2006, 2010), three FA Cup wins (2007, 2009, 2010) and two Football League Cup conquests (2005, 2007).
Then Villas-Boas happened.

Coming from FC Porto, Villas-Boas attempted to play the 4-3-3 in his first few matches as Blues’ boss, but as results began to sour, and his high line of defence was being abused, the Portuguese decided to alter the Chelsea ethic and play with two holding midfielders. Regardless of his adjustments, the proverbial cat was already amongst the pigeons and Villas-Boas was sacked—but his tactics were not. 

These successes, while sweet, were a bit of a misnomer. The tactics one could argue were irrelevant. What carried Chelsea to their 2012 Double was camaraderie, experience, intestinal fortitude and, some might say, a hint of destiny. The Lampards and Essiens of the world were not the box-to-box midfielders they once were—due to age and injury—so the holding midfield roles seemed to make sense. Add Juan Mata plus the emerging crop of attacking-midfield talent and the 4-2-3-1 has been Chelsea’s modus operandi henceforth. However, the tactic should not be as automatic as it was preceding Mourinho’s second Chelsea spell.

An easy issue to point out with the current Chelsea squad is the lack of striking proficiency. "Fernando Torres, Demba Ba and Samuel Eto'o are not good enough to win you silverware" the argument may go. While the trio might be part of the problem, the biggest issue lies with how Chelsea play football. Condemning star players—who played as such before their London moves—as now "garbage" is supremely unfair.

Are we to believe Torres and Ba forgot how to play football all of a sudden? That Chelsea's blue kit is some version of striking kryptonite? Those seem like giant leaps. 
What is more plausible are the three players behind Chelsea's "No. 9" are choking their forward's supply line. Chelsea's midfield gobbles attempts on goal, attempts which would normally fall to a team’s primary striker; the thought Roman Abramovich spending £35-60 million on Diego Costa or Edinson Cavani would solve the problem is too simple a notion.
How then is this issue resolved?

The best solution would be to drop Oscar deeper and play a 4-3-3—which might be more accurately envisioned as 4-1-2-3.
During Chelsea's 2013 Asian summer tour, Mourinho spoke about his midfielders' proclivities, via The Guardian's Dominic Fifield:

This is a team where we have not one or two but three or four players who like very much to be a "No. 10." [Kevin] De Bruyne likes it, Oscar likes it, Mata likes it, [Eden] Hazard likes it. It is a natural system for all these players to play.

The Portuguese sold Mata to Manchester United, Kevin De Bruyne to VfL Wolfsburg and has Hazard accustom to playing wider, rendering three of Mourinho's four pre-season "No. 10s" unavailable. If Chelsea's strikers are not firing in the number of goals required and the players meant to feed them are not in positions (or the mindset) to do so—a change is needed.

Instead of "attacking midfielders," Willian, Hazard, Schurrle and Mohamed Salah (with the possible additions of Victor Moses and Marko Marin) would play as "wingers" in a 4-3-3.
While the distinction between “attacking midfielder” and “winger” could be seen as semantic—the roles are quite different. Getting their boots white with chalk—creating width—would allow their teammates space to run into and find gaps rather than clutter the middle of the pitch.
For the two central midfielders, Ramires and Oscar (or possibly a summer transfer like Ross Barkley / the returning Marko van Ginkel) would serve as lynchpins. Either there to stifle opponents or launch attacks with their respective attacking options.

The notion a team of Chelsea's quality need two holding midfielders against the likes of Norwich City or Cardiff City seems rather embarrassing. The idea has become a norm because Villas-Boas was too prideful to play a lower block with his defensive line—rendering another body in defensive midfield necessary.
Pre-Mourinho, Hazard, Oscar and Mata were relatively quiet in the defensive third of the pitch. Once the Portuguese arrived, players were sold who did not fit the mold but others have adopted their manager's defensive mindedness. The determination to track back and help their fullbacks should be more than enough to offset a loss in the pivot—especially with three central midfielders. Liverpool's Daniel Sturridge and Luis Suarez are on pace to break the Premier League "record" of most goals scored by a tandem. The current 38-game season leaders are Lampard (22) and Drogba (29) from 2009-10.

The west London outfit set the Premiership's goals-in-a-season record (103) that campaign—leading to their fourth top-flight success. Not coincidentally, 2009-10 was the last season a Chelsea player scored over 15 goals in the Premier League.
The Blues' formation you ask?

Ancelotti's 4-3-3.

If Mourinho is frustrated by Chelsea's lack of goalscoring, a measure of blame should be given to the strikers, but double that to the tactics. The Blues' boss has adopted the same formation and players who have finished sixth and third the past two seasons; Chelsea have not exactly been setting the Premier League alight over that time.
The Portuguese used variations of the 4-3-3 with vast success in Portugal and England but converted to the 4-2-3-1 during his reign at Internazionale. You cannot blame him for the switch in methods, as Mourinho won the Treble with Inter Milan in 2009-10.

During his three seasons at Real Madrid, Mourinho was a wizard of sorts. The Spanish giants dabbled with various formational adaptations in an attempt to give Cristiano Ronaldo and Co. attacking freedom while remaining defensively sound.

Coming back to a Chelsea squad already familiar with the 4-2-3-1 must have seemed a simple transition but it has proven a hindrance in the goal department.
Chelsea's talent demands winning in any formation but merely getting results are what Southampton and Newcastle United are doing at the moment; Chelsea are in the business of competing for titles. While not a "Manchester United" situation, the Blues are lacking the firepower you need to sustain a proper title race.
The seemingly "lesser matches" against lower-table sides prove just as valuable in the season's final tally. When point-needy teams park their buses, goal scorers are needed to get three points; Chelsea may look to Crystal Palace (A), Aston Villa (A) and West Ham United (H) as eight points that—with a goal-scoring striker—could have seen them home and dry at this stage in the title race.

With that in mind, instead of replicating the worn ideas of Villas-Boas, Di Matteo and Benitez, the Special-Happy-Frustrated One should reinstall (or at the very least experiment with) the 4-3-3 which saw him win back-to-back Premier League titles in 2005 and 2006. The fact Chelsea have at least six outfield players (Ba, Ashley Cole, Eto'o, Lampard, Mikel and Torres) who are ripe for replacing bodes well. For Stamford Bridge's sake, let us hope Chelsea's incoming talent is paired with a change in philosophy.