The Portuguese may well be the "Happy One" these days, yet he remains as ruthless as ever. Regardless of the transfer fee Chelsea received by selling Mata—£37.1 million, per United's official site—it reaffirms the notion that players either sink or swim where Mourinho is concerned.
It wasn't Mata's attitude or ability that saw him depart West London; it was more his style of play and what he offers the team in other areas other than just his preferred role behind the striker.
Chelsea fans are understandably mourning his exit—the Spaniard was named Player of the Year in his two full seasons with the Blues, after all—but they need not worry about further casualties. Mata's transfer is the end of it; the other emerging heroes at Stamford Bridge will not be going anywhere anytime soon.
Why? Because they are all Mourinho players. Oscar has demonstrated that much in 2013-14, being the man around whom the new Chelsea revolves. And alongside him is Eden Hazard.
The Belgian was a revelation last term, giving Chelsea fans enough in his debut season to show why the club were so eager to beat the likes of Real Madrid to his signature. He may have cost £32 million, but from the moment he slipped into Chelsea blue, it hasn't been difficult to see why.
We all remember the impact he made in the opening few games of 2012-13. Embarrassing the Wigan Athletic defence on his debut, Hazard had fans in West London dreaming he would deliver a fifth league title to Stamford Bridge.
The Belgian took just two minutes to set up Branislav Ivanovic to open Chelsea's account for the season before winning a penalty for Frank Lampard, who doubled Chelsea's lead just five minutes later.
It remained that way until the final whistle, with Chelsea winning 2-0. It was promising stuff, but more would follow with two assists in the following game against Reading, as Chelsea won 4-2. Days later, he all but dismantled Newcastle United single-handedly, scoring himself before teeing up Fernando Torres in a 2-0 win.
Chelsea were looking like a good bet to finish the season as champions, although those who predicted the Blues going all the way were somewhat naive in their analysis, with the young starlets in Chelsea's lineup suffering a dramatic loss of form—Hazard among them.
With a young team and relatively young manager in Roberto Di Matteo, Chelsea lost their way. So, too, did Hazard. The 22-year-old went missing at Stamford Bridge, failing to impact games the way he had earlier in the campaign.
He still looked sharp, but the end product was no longer there. From mid-October through to Dec. 19, when Chelsea defeated Leeds United, 5-1, in the Capital One Cup, Hazard scored just twice and chipped in with one assist.
During that dip in form, Chelsea's title chances faded dramatically. What's more, they were eliminated from the Champions League and Di Matteo was sacked and replaced by Rafa Benitez.
Once the man many thought could help inspire Chelsea to glory, Hazard was proving to be the contrary: another player looking to others for inspiration and in need of a little of it himself.
Sure, there were glimpses of what he could produce, but he still lacked that killer instinct and consistency that would rank him among Europe's finest.
Enter Jose Mourinho.
Mourinho is famed for getting every ounce of talent from his players, squeezing it from them in training and, most importantly, in matches. Now, he's doing it with Hazard.
Stats can often be misleading and not give us the full picture. When comparing Hazard's 2013-14 figures with last season's, however, there is little doubt as to his evolution as a player.
Mourinho has been asking for more from Chelsea this season—more goals, more commitment and more ruthlessness in front of goal. Of all those who have answered the manager's call, it's Hazard who has looked most impressive.
After 34 Premier League appearances in 2012-13, Hazard hit a respectable nine goals. This year, he has reached that tally after just 22 and is showing no sign of stopping.
The Belgian has 11 goals in all competitions, putting him on top of the club's scoring charts. Not only that, but he is winning games and points at vital moments.
Take the West Bromwich Albion draw in November. Trailing 2-1 with the referee ready to blow his whistle, Ramires won a penalty—a penalty that Hazard stepped up to convert with confidence. Not only did he earn his team a point, but he continued Mourinho's unique record of having never lost a Premier League home match in the process.
More recently against Hull City, it was a moment of genius from Hazard that unlocked a stubborn Tigers defence to get Chelsea on the front foot. And during the Christmas period, Hazard proved the difference against Swansea City before his sublime equalizer against Liverpool gave Chelsea the momentum to win the game.
It could have been very different after he was dropped from the lineup against Schalke in November, however, having missed training after an impromptu trip to France to watch his former team, Lille.
Hazard received a public dressing-down from Mourinho, per Dominic Fifield of The Guardian, but since then, he has been one of Chelsea's best players—a leader.
"It’s more because it’s Mourinho—that’s a bit scary for everyone," he said in a December interview with The Telegraph's Jason Burt, admitting he is scared of his manager when it comes to criticism.
Mourinho's ability to frighten his player certainly hasn't proved a bad thing. In fact, it has helped Hazard focus in a way we haven't seen before.
After being a younger member of the squad who looked up to those around him, we're now suddenly seeing a mature individual who is taking the reins from his older, more experienced colleagues.
For some time, the baton has needed to be passed from Chelsea's previous generation to the next, and Hazard is showing that process may have finally begun.
Garry Hayes is Bleacher Report's lead Chelsea correspondent and will be following the club from a London base throughout the 2013-14 season. Follow him on Twitter here @garryhayes