The 23-year-old World Cup winner was the subject of a club-record £18 million bid from Arsenal earlier in the transfer window, but it appears the appeal of the club just doesn't compare to Chelsea.
Liverpool and Tottenham Hotspur were also credited with an interest, while he was reportedly set for a move to FC Barcelona if they didn't sign Alexis Sanchez, after having almost moved to the Camp Nou last season in a reported £17.5 million deal.
He now finds himself as part of the new setup at Stamford Bridge, with fans crying out for a new playmaker to replace Frank Lampard and a new winger to supplement Florent Malouda.
Mata plays in both positions, but could well be used through the middle, especially considering Nicolas Anelka has done well on the wings (scoring against West Brom) and a deal for Tottenham's Luka Modric has seemingly stagnated.
It makes sense, as Chelsea already have good talents on the flanks and have been seeking an attacking midfielder as their top transfer target.
It also makes sense in that playing through the middle, Mata can best help compatriot Fernando Torres score goals—something the £50 million January signing has been deleteriously failing to do in recent months.
The 27-year-old striker generally scores more goals when able to attack the penalty area through the centre, but has lacked good supply from central midfield ever since joining the club.
And that's where Mata comes in. The former Valencia man was one of the best passers in La Liga last season, and is one of the main reasons why Roberto Soldado managed to clock up 25 goals throughout the campaign, and why out-of-favour Aritz Aduriz scored 10 goals from just 20 starts.
Villas-Boas knows if he doesn't win trophies, his only saving grace is getting serious value for money out of Torres, and if Mata's La Liga statistics can be just as good in the Premier League, it could well happen.
European U-21 champion Mata was the key playmaker for Valencia in 2010/11, hitting 74 key passes at an average of 2.24 per game.
And to prove just how good those key passes were, Mata made 12 league assists last season, equating to an assist every 6.16 key passes, the most efficient ratio in La Liga and even better than Barcelona's Xavi Hernandez.
To put his passing in perspective, Mata made more key passes than any Chelsea player barring one, Florent Malouda.
However, while Malouda made an incredible 117 key passes, he only made four assists at a rate of one assist every 29.25 key passes.
Mata also has a higher pass completion rate than Malouda and any Chelsea player, having completed 86 per cent of his passes.
The Spaniard's stats also make for better reading than those of Luka Modric.
Modric has the same pass completion rate as Mata, and overall made more successful passes, but only made 64 key passes in virtually the same amount of matches and finished with a paltry two assists in the Premier League.
Considering the new signing from Valencia only made 0.7 successful dribbles per game last season, fewer than Florent Malouda and several other Chelsea players, it's clear he's been signed for his passing and ability to create goalscoring chances more than anything.
And considering Fernando Torres is an expensive striker lacking confidence and goals, it's evident Juan Mata has been signed to get the best of him, Chelsea and Andre Villas-Boas' hopes of keeping his job.