The £21 million midfielder has transformed Jose Mourinho's Premier League leaders.
Chelsea have made the transfer market look easy in 2014.
One may argue that they have enough money and influence to guarantee top signings in ways other clubs can't - but that's not really true, is it?
Look at Manchester City and Manchester United, for instance. Both have attracted players with big names and big wages. Both have recruited badly (City overall and United in their failure to address their real weaknesses).
The same goes for Arsenal, who have simply ignored key areas, and Liverpool, who have just spent unwisely. If it's so easy to do, then, why isn't everyone buying a Diego Costa and Cesc Fabregas in the Premier League?
But, in truth, Chelsea's most important signing of 2014 did not come in the summer. Their most influential new recruit moved to London in January - in holding midfielder Nemanja Matic.
|Tackles Won||Interceptions Won||Appearances||Minutes Played|
As Opta stats (taken before today's clash with West Brom) show, Matic seems to have made it his mission to top the tackles and interceptions won charts since joining Chelsea, starring in those departments this term just like he did the last.
It must be noted that Cesar Azpilicueta may have overtaken the Serb had he played as many minutes but that takes nothing away from the midfielder's impact.
And, stats aside, it is Matic's solidity that allows his team-mates to flourish going forward. The 26-year-old has particularly allowed Fabregas creative freedom - meaning the Spaniard's signing would not look half as good without the Serb already in the team alongside him.
Since Matic moved to Stamford Bridge, Chelsea have simply looked a different team. Without him, the Blues would be yet another forward-thinking side without the strength in the middle to back it up. With him, Mourinho's men are a class apart from their rivals.